(1)

New Mountain Finance Advisers BDC, L.L.C. (the "Investment Adviser") has agreed to bear $1,575,000, or $0.42 per share, of the sales load in connection with this offering, which is reflected in the above table. In addition, the Investment Adviser has agreed to pay to the underwriters an additional supplemental payment of $675,000, or $0.18 per share, which reflects the difference between the public offering price and the proceeds per share received by us in this offering. All payments made by the Investment Adviser will not be subject to reimbursement by us. All other expenses of the offering will be borne by us. We will incur approximately $0.3 million of estimated expenses in connection with this offering.

(2)

To the extent that the underwriters sell more than 3,750,000 shares of our common stock, the underwriters have the option to purchase up to an additional 562,500 shares of our common stock at the public offering price, less the sales load, within 30 days of the date of this prospectus supplement. If the underwriters exercise this option in full, the total public offering price, additional supplemental payment by the Investment Adviser, proceeds to us and sales load payable by the Investment Adviser will be $58,520,625, $776,250, $59,296,875 and $1,811,250, respectively. See "Underwriting".

(3)

See "Underwriting" for details of compensation to be received by the underwriters.

PROSPECTUS SUPPLEMENT SUMMARY

          This summary highlights some of the information in this prospectus supplement and the accompanying prospectus. It may not contain all the information that is important to you. For a more complete understanding, we encourage you to read this entire prospectus supplement and the accompanying prospectus and the documents to which we have referred in this prospectus supplement, together with the accompanying prospectus, including the risks set forth under "Risk Factors" and the other information included in this prospectus supplement and the accompanying prospectus.

          In this prospectus supplement, unless the context otherwise requires, references to:

•

"NMFC", the "Company", "we", "us" and "our" refers to New Mountain Finance Corporation, a Delaware corporation, which was incorporated on June 29, 2010, including, where appropriate, its wholly-owned direct and indirect subsidiaries;



•

"NMF Holdings" and "Predecessor Operating Company" refers to New Mountain Finance Holdings, L.L.C., a Delaware limited liability company.;



•

"NMF SLF" refers to New Mountain Finance SPV Funding, L.L.C., a Delaware limited liability company;



•

"NMNLC" refers to New Mountain Net Lease Corporation, a Maryland corporation;



•

"NMFDB" refers to New Mountain Finance DB, L.L.C., a Delaware limited liability company;



•

"SBIC I GP" refers to New Mountain Finance SBIC G.P. L.L.C., a Delaware limited liability company;



•

"SBIC I" refers to New Mountain Finance SBIC L.P., a Delaware limited partnership;



•

"SBIC II GP" refers to New Mountain Finance SBIC II G.P. L.L.C., a Delaware limited liability company;



•

"SBIC II" refers to New Mountain Finance SBIC II L.P., a Delaware limited partnership;



•

"Guardian AIV" refers to New Mountain Guardian AIV, L.P.;



•

"AIV Holdings" refers to New Mountain Finance AIV Holdings Corporation, a Delaware corporation which was incorporated on March 11, 2011, of which Guardian AIV was the sole stockholder;



•

"Investment Adviser" refers to New Mountain Finance Advisers BDC, L.L.C., our investment adviser;



•

"Administrator" refers to New Mountain Finance Administration, L.L.C., our administrator;



•

"New Mountain Capital" refers to New Mountain Capital Group, L.P. together with New Mountain Capital L.L.C. and its affiliates whose ultimate owners include Steven B. Klinsky and other related vehicles;



•

"Predecessor Entities" refers to New Mountain Guardian (Leveraged), L.L.C. and New Mountain Guardian Partners, L.P., together with their respective direct and indirect wholly-owned subsidiaries prior to our initial public offering;



•

"NMFC Credit Facility" refers to our Senior Secured Revolving Credit Agreement with Goldman Sachs Bank USA, Morgan Stanley Bank, N.A. and Stifel Bank & Trust, dated June 4, 2014, as amended (together with the related guarantee and security agreement);

•

"Holdings Credit Facility" refers to NMF Holdings' Third Amended and Restated Loan and Security Agreement with Wells Fargo Bank, National Association, dated October 24, 2017, as amended;



•

"DB Credit Facility" refers to our Secured Revolving Credit Agreement with Deutsche Bank AG, New York Branch, dated December 14, 2018;



•

"Predecessor Holdings Credit Facility" refers to NMF Holdings' Amended and Restated Loan and Security Agreement with Wells Fargo Bank, National Association, dated May 19, 2011, as amended;



•

"SLF Credit Facility" refers to NMF SLF's Loan and Security Agreement with Wells Fargo Bank, National Association, dated October 27, 2010, as amended;



•

"2014 Convertible Notes" refers to our 5.00% convertible notes due June 15, 2019 issued on June 3, 2014 and September 30, 2016 under an indenture dated June 3, 2014, between us and U.S. Bank National Association, as trustee;



•

"2016 Unsecured Notes" refers to our 5.313% unsecured notes due May 15, 2021 issued on May 6, 2016 and September 30, 2016 to institutional investors in a private placement;



•

"2017A Unsecured Notes" refers to our 4.760% unsecured notes due July 15, 2022 issued on June 30, 2017 to institutional investors in a private placement;



•

"2018A Unsecured Notes" refers to our 4.870% unsecured notes due January 30, 2023 issued on January 30, 2018 to institutional investors in a private placement;



•

"2018B Unsecured Notes" refers to our 5.36% unsecured notes due June 28, 2023 issued on July 5, 2018 to institutional investors in a private placement;



•

"2018 Convertible Notes" refers to our 5.75% convertible notes due August 15, 2023 issued on August 20, 2018 and August 30, 2018 under an indenture and a first supplemental indenture, both dated August 20, 2018, between us and U.S. Bank National Association, as trustee;



•

"5.75% Unsecured Notes" refers to our 5.75% unsecured notes due October 1, 2023, issued on September 25, 2018 and October 17, 2018 under an indenture, dated August 20, 2018, as supplemented by a second supplemental indenture thereto, dated September 25, 2018, between us and U.S. Bank National Association, as trustee; and



•

"Unsecured Notes" refers to the 2016 Unsecured Notes, the 2017A Unsecured Notes, the 2018A Unsecured Notes, 2018B Unsecured Notes and the 5.75% Unsecured Notes.

          For the periods prior to and as of December 31, 2013, all financial information provided in this prospectus supplement and the accompanying prospectus reflect our organizational structure prior to the restructuring on May 8, 2014 described under "Description of Restructuring" in the accompanying prospectus, where NMF Holdings functioned as the operating company.

Overview

New Mountain Finance Corporation

          We are a Delaware corporation that was originally incorporated on June 29, 2010 and completed our initial public offering ("IPO") on May 19, 2011. We are a closed-end, non-diversified management investment company that has elected to be regulated as a business development company ("BDC") under the Investment Company Act of 1940, as amended (the "1940 Act"). We have elected to be treated, and intend to comply with the requirements to continue to qualify annually, as a regulated investment company ("RIC") under Subchapter M of the Internal Revenue

Code of 1986, as amended (the "Code"). NMFC is also registered as an investment adviser under the Investment Advisers Act of 1940, as amended (the "Advisers Act").

          The Investment Adviser is a wholly-owned subsidiary of New Mountain Capital. New Mountain Capital is a firm with a track record of investing in the middle market. New Mountain Capital focuses on investing in defensive growth companies across its private equity, public equity and credit investment vehicles. The Investment Adviser manages our day-to-day operations and provides us with investment advisory and management services. The Administrator, a wholly-owned subsidiary of New Mountain Capital, provides the administrative services necessary to conduct our day-to-day operations.

          Our wholly-owned subsidiary, NMF Holdings, is a Delaware limited liability company whose assets are used to secure NMF Holdings' credit facility. For additional information about our organizational structure prior to May 8, 2014, see "Description of Restructuring" in the accompanying prospectus. NMF Ancora Holdings Inc. ("NMF Ancora"), NMF QID NGL Holdings, Inc. ("NMF QID") and NMF YP Holdings Inc. ("NMF YP"), our wholly-owned subsidiaries, are structured as Delaware entities that serve as tax blocker corporations which hold equity or equity-like investments in portfolio companies organized as limited liability companies (or other forms of pass-through entities). We consolidate our tax blocker corporations for accounting purposes. The tax blocker corporations are not consolidated for income tax purposes and may incur income tax expense as a result of their ownership of the portfolio companies. Additionally, our wholly-owned subsidiary, New Mountain Finance Servicing, L.L.C. ("NMF Servicing"), serves as the administrative agent on certain investment transactions. SBIC I, and its general partner, SBIC I GP, are organized in Delaware as a limited partnership and limited liability company, respectively. During the year ended December 31, 2017, SBIC II and its general partner, SBIC II GP, were organized in Delaware as a limited partnership and limited liability company, respectively. SBIC I, SBIC I GP, SBIC II and SBIC II GP are our consolidated wholly-owned direct and indirect subsidiaries. SBIC I and SBIC II each received a license from the United States ("U.S.") Small Business Administration (the "SBA") to operate as a small business investment company ("SBIC") under Section 301(c) of the Small Business Investment Act of 1958, as amended (the "1958 Act"). Our wholly-owned subsidiary, NMNLC, a Maryland corporation, was formed to acquire commercial real properties that are subject to "triple net" leases and has qualified and intends to continue to qualify as a real estate investment trust ("REIT") within the meaning of Section 856(a) of the Code. During the year ended December 31, 2018, NMFDB was organized in Delaware as a limited liability company whose assets are used to secure NMFDB's credit facility.

          Our investment objective is to generate current income and capital appreciation through the sourcing and origination of debt securities at all levels of the capital structure, including first and second lien debt, notes, bonds and mezzanine securities. The first lien debt may include traditional first lien senior secured loans or unitranche loans. Unitranche loans combine characteristics of traditional first lien senior secured loans as well as second lien and subordinated loans. Unitranche loans will expose us to the risks associated with second lien and subordinated loans to the extent we invest in the "last out" tranche. In some cases, our investments may also include equity interests. Our primary focus is in the debt of defensive growth companies, which are defined as generally exhibiting the following characteristics: (i) sustainable secular growth drivers, (ii) high barriers to competitive entry, (iii) high free cash flow after capital expenditure and working capital needs, (iv) high returns on assets and (v) niche market dominance. Similar to us, SBIC I's and SBIC II's investment objectives are to generate current income and capital appreciation under our investment criteria. However, SBIC I's and SBIC II's investments must be in SBA eligible small businesses. Our portfolio may be concentrated in a limited number of industries. As of September 30, 2018, our top five industry concentrations were business services, software, healthcare services, education and investment funds.

          The investments that we invest in are almost entirely rated below investment grade or may be unrated, which are often referred to as "leveraged loans", "high yield" or "junk" debt investments, and may be considered "high risk" or speculative compared to debt investments that are rated investment grade. Such issuers are considered more likely than investment grade issuers to default on their payments of interest and principal, and such risk of default could reduce our net asset value and income distributions. Our investments are also primarily floating rate debt investments that contain interest reset provisions that may make it more difficult for borrowers to make debt repayments to us if interest rates rise. In addition, some of our debt investments will not fully amortize during their lifetime, which could result in a loss or a substantial amount of unpaid principal and interest due upon maturity. Our debt investments may also lose significant market value before a default occurs. Furthermore, an active trading market may not exist for these securities. This illiquidity may make it more difficult to value our investments.

          As of September 30, 2018, our net asset value was $1,033.5 million and our portfolio had a fair value of approximately $2,294.8 million in 92 portfolio companies, with a weighted average yield to maturity at cost for income producing investments ("YTM at Cost") of approximately 11.0% and a weighted average yield to maturity at cost for all investments ("YTM at Cost for Investments") of approximately 10.9%. The YTM at Cost calculation assumes that all investments, including secured collateralized agreements, not on non-accrual are purchased at cost on the quarter end date and held until their respective maturities with no prepayments or losses and exited at par at maturity. The YTM at Cost for Investments calculation assumes that all investments, including secured collateralized agreements, are purchased as cost on the quarter end date and held until their respective maturities with no prepayments or losses and exited at par at maturity. YTM at Cost and YTM at Cost for Investments calculations exclude the impact of existing leverage. YTM at Cost and YTM at Cost for Investments uses the London Interbank Offered Rate ("LIBOR") curves at each quarter's end date. The actual yield to maturity may be higher or lower due to the future selection of the LIBOR contracts by the individual companies in our portfolio or other factors.

Recent Developments

Originations and Repayments

          We had approximately $332.0 million of originations and commitments since September 30, 2018 through February 8, 2019. This was offset by approximately $79.2 million of repayments and $119.1 million of sales during the same period.

5.75% Unsecured Notes

          On October 17, 2018, in connection with the registered public offering, we issued an additional $1.8 million aggregate principal amount of the 5.75% Unsecured Notes pursuant to the exercise of an overallotment option by the underwriters of the 5.75% Unsecured Notes.

Distribution

          On November 1, 2018, our board of directors declared a fourth quarter 2018 distribution of $0.34 per share which was paid on December 28, 2018 to holders of record as of December 14, 2018.

Holdings Credit Facility

          On November 19, 2018, we entered into the Second Amendment to Loan and Security Agreement (the "Second Amendment"), which amended the Holdings Credit Facility. Among other changes, the Second Amendment: increased the maximum facility amount from $495.0 million to $695.0 million; made certain technical changes to facilitate further increases should any new or existing lender and NMF Holdings mutually agree to do so; modified certain eligibility criteria and concentration limits for loans acquired by NMF Holdings; lowered the concentration limit for non-first lien loans from 50% to 35%; changed the minimum asset coverage ratio for us and our consolidated subsidiaries from 2:1 to 1.5:1, changed the cure level to 1.75:1; and changed the default basket for tax liens and other governmental liens from $50,000 to $250,000.

          On December 13, 2018, January 8, 2019 and January 25, 2019, NMF Holdings entered into certain Joinder Supplements (the "Joinders") to add TIAA, FSB, Old Second National Bank and Sumitomo Mitsui Trust Bank, Limited, New York, respectively, as new lenders under the Holdings Credit Facility. After giving effect to each of the Joinders, the aggregate commitments of the lenders under the Holdings Credit Facility equals $675.0 million. The Holdings Credit Facility continues to have a revolving period ending on October 24, 2020, and will still mature on October 24, 2022.

Deutsche Bank Facility

          On December 14, 2018, our newly formed wholly-owned subsidiary, NMFDB, as the borrower (the "DB Facility Borrower"), entered into a secured revolving credit facility with Deutsche Bank AG, New York Branch ("Deutsche Bank") pursuant to a Loan Financing and Servicing Agreement (together with the exhibits and schedules thereto, the "DB Facility Agreement" and the secured revolving credit facility thereunder, the "DB Credit Facility"), by and among the DB Facility Borrower, us, as equityholder and servicer, the lenders from time to time party thereto, Deutsche Bank, as the facility agent (the "Facility Agent"), the other agents from time to time party thereto and U.S. Bank National Association, as collateral agent and collateral custodian. The facility amount and the commitment of Deutsche Bank is $100.0 million. With the consent of the Facility Agent (which may be made subject to conditions), Deutsche Bank and the DB Facility Borrower may in the future agree to increase the commitments and the facility amount by up to $200.0 million or add additional lenders. The lenders under the DB Credit Facility will make advances to the DB Facility Borrower during a revolving period (the "Revolving Period") that will expire on December 14, 2021, provided that the Revolving Period may be extended with the consent of the lenders and may also terminate early if an event of default or other adverse events, specified in the DB Facility Agreement, occur. The maturity date for the DB Credit Facility is December 14, 2023.

Preliminary Estimates of Net Asset Value and Net Investment Income

          Set forth below is a preliminary estimate of our net asset value per share as of December 31, 2018 and a preliminary estimate of our net investment income per share range for the three months ended December 31, 2018. The following estimates are not a comprehensive statement of our financial condition or results for the period ended December 31, 2018. We advise you that our actual results for the three months ended December 31, 2018 may differ materially from these estimates, which are given only as of the date of this prospectus supplement, as a result of the completion of our financial closing procedures, final adjustments and other developments, including changes in interest rates, changes in the businesses to whom we have made loans or market and industry fluctuations, which may arise between now and the time that our financial results for the three months ended December 31, 2018 are finalized. This information is inherently uncertain.

          As of the date of this prospectus supplement, we estimate that our net asset value per share as of December 31, 2018 was approximately $13.20 to $13.25. We also estimate that as of the date of this prospectus supplement our net asset value per share has increased since December 31, 2018 due to changes in market conditions. The offering price per share of our common stock, net of the sales load (underwriting discounts and commissions), will be in excess of the net asset value per share of our common stock at the time we make this offering.

          As of the date of this prospectus supplement, we currently expect that our net investment income per share was between $0.35 and $0.36 for the three months ended December 31, 2018.

          The preliminary financial estimates provided herein have been prepared by, and are the responsibility of, management. Neither Deloitte & Touche LLP, our independent registered public accounting firm, nor any other independent accountants have audited, reviewed, compiled, or performed any procedures with respect to the accompanying preliminary financial data.

Accordingly, Deloitte & Touche LLP does not express an opinion or any form of assurance with respect thereto and assumes no responsibility for, and disclaims any association with, this information.

Share Repurchase Program

          On January 3, 2019, our board of directors extended our common stock repurchase program and we expect our common stock repurchase program to be in place until the earlier of December 31, 2019 or until $50.0 million of our outstanding shares of common stock have been repurchased.

The Investment Adviser

          The Investment Adviser, a wholly-owned subsidiary of New Mountain Capital, manages our day-to-day operations and provides us with investment advisory and management services. In particular, the Investment Adviser is responsible for identifying attractive investment opportunities, conducting research and due diligence on prospective investments, structuring our investments and monitoring and servicing our investments. We currently do not have, and do not intend to have, any employees. The Investment Adviser also manages New Mountain Guardian Partners II, L.P., a Delaware limited partnership, and New Mountain Guardian Partners II Offshore, L.P., a Cayman Islands exempted limited partnership, (together "Guardian II"), which commenced operations in April 2017. As of September 30, 2018, the Investment Adviser was supported by over 140 employees and senior advisors of New Mountain Capital.

          The Investment Adviser is managed by a five member investment committee (the "Investment Committee"), which is responsible for approving purchases and sales of our investments above $10.0 million in aggregate by issuer. The Investment Committee currently consists of Steven B. Klinsky, Robert A. Hamwee, Adam B. Weinstein and John R. Kline. The fifth and final member of the Investment Committee will consist of a New Mountain Capital Managing Director who will hold the position on the Investment Committee on an annual rotating basis. Beginning in August 2018, Andre V. Moura was appointed to the Investment Committee for a one year term. In addition, our executive officers and certain investment professionals of the Investment Adviser are invited to all Investment Committee meetings. Purchases and dispositions below $10.0 million may be approved by our Chief Executive Officer. These approval thresholds are subject to change over time. We expect to benefit from the extensive and varied relevant experience of the investment professionals serving on the Investment Committee, which includes expertise in private equity, primary and secondary leveraged credit, private mezzanine finance and distressed debt.

Competitive Advantages

          We believe that we have the following competitive advantages over other capital providers to middle market companies:

Proven and Differentiated Investment Style With Areas of Deep Industry Knowledge

          In making its investment decisions, the Investment Adviser applies New Mountain Capital's long-standing, consistent investment approach that has been in place since its founding in 1999. We focus on companies in defensive growth niches of the middle market space where we believe few debt funds have built equivalent research and operational size and scale.

          We benefit directly from New Mountain Capital's private equity investment strategy that seeks to identify attractive investment sectors from the top down and then works to become a well positioned investor in these sectors. New Mountain Capital focuses on companies and industries with sustainable strengths in all economic cycles, particularly ones that are defensive in nature, that

have secular tailwinds and can maintain pricing power in the midst of a recessionary and/or inflationary environment. New Mountain Capital focuses on companies within sectors in which it has significant expertise (examples include software, education, niche healthcare, business services, federal services and distribution & logistics) while typically avoiding investments in companies with products or services that serve markets that are highly cyclical, have the potential for long-term decline, are overly-dependent on consumer demand or are commodity-like in nature.

          In making its investment decisions, the Investment Adviser has adopted the approach of New Mountain Capital, which is based on three primary investment principles:

1.

A generalist approach, combined with proactive pursuit of the highest quality opportunities within carefully selected industries, identified via an intensive and structured ongoing research process;



2.

Emphasis on strong downside protection and strict risk controls; and



3.

Continued search for superior risk adjusted returns, combined with timely, intelligent exits and outstanding return performance.

Experienced Management Team and Established Platform

          The Investment Adviser's team members have extensive experience in the leveraged lending space. Steven B. Klinsky, New Mountain Capital's Founder, Chief Executive Officer and Managing Director and Chairman of our board of directors, was a general partner of Forstmann Little & Co., a manager of debt and equity funds totaling multiple billions of dollars in the 1980s and 1990s. He was also a co-founder of Goldman Sachs & Co. LLC's Leverage Buyout Group in the period from 1981 to 1984. Robert A. Hamwee, our Chief Executive Officer and Managing Director of New Mountain Capital, was formerly President of GSC Group, Inc. ("GSC"), where he was the portfolio manager of GSC's distressed debt funds and led the development of GSC's CLOs. John R. Kline, our President and Chief Operating Officer and Managing Director of New Mountain Capital, worked at GSC as an investment analyst and trader for GSC's control distressed and corporate credit funds and at Goldman Sachs & Co. LLC in the Credit Risk Management and Advisory Group.

          Many of the debt investments that we have made to date have been in the same companies with which New Mountain Capital has already conducted months of intensive acquisition due diligence related to potential private equity investments. We believe that private equity underwriting due diligence is usually more robust than typical due diligence for loan underwriting. In its underwriting of debt investments, the Investment Adviser is able to utilize the research and hands-on operating experience that New Mountain Capital's private equity underwriting teams possess regarding the individual companies and industries. Business and industry due diligence is led by a team of investment professionals of the Investment Adviser that generally consists of three to seven individuals, typically based on their relevant company and/or industry specific knowledge. Additionally, the Investment Adviser is also able to utilize its relationships with operating management teams and other private equity sponsors. We believe this differentiates us from many of our competitors.

Significant Sourcing Capabilities and Relationships

          We believe the Investment Adviser's ability to source attractive investment opportunities is greatly aided by both New Mountain Capital's historical and current reviews of private equity opportunities in the business segments we target. To date, a significant majority of the investments that we have made are in the debt of companies and industry sectors that were first identified and reviewed in connection with New Mountain Capital's private equity efforts, and the majority of our current pipeline reflects this as well. Furthermore, the Investment Adviser's investment professionals have deep and longstanding relationships in both the private equity sponsor community and the lending/agency community which they have and will continue to utilize to generate investment opportunities.

Risk Management through Various Cycles

          New Mountain Capital has emphasized tight control of risk since its inception. To date, New Mountain Capital has never experienced a bankruptcy of any of its portfolio companies in its private equity efforts. The Investment Adviser seeks to emphasize tight control of risk with our investments in several important ways, consistent with New Mountain Capital's historical approach. In particular, the Investment Adviser:

•

Emphasizes the origination or purchase of debt in what the Investment Adviser believes are defensive growth companies, which are less likely to be dependent on macro-economic cycles;



•

Targets investments in companies that are preeminent market leaders in their own industries, and when possible, investments in companies that have strong management teams whose skills are difficult for competitors to acquire or reproduce; and



•

Targets investments in companies with significant equity value in excess of our debt investments.

Access to Non Mark to Market, Seasoned Leverage Facility

          The amount available under the Holdings Credit Facility and the DB Credit Facility are generally not subject to reduction as a result of mark to market fluctuations in our portfolio investments. None of our credit facilities mature prior to June 2022. For a detailed discussion of our credit facilities, see "Management's Discussion and Analysis of Financial Conditions and Results of Operations — Liquidity and Capital Resources" in this prospectus supplement.

Market Opportunity

          We believe that the size of the market for investments that we target, coupled with the demands of middle market companies for flexible sources of capital at competitive terms and rates, create an attractive investment environment for us.

•

Large pool of uninvested private equity capital available for new buyouts.  We expect that private equity firms will continue to pursue acquisitions and will seek to leverage their equity investments with mezzanine loans and/or senior loans (including traditional first and second lien, as well as unitranche loans) provided by companies such as ours.



•

The leverage finance market has a high level of financing needs over the next several years due to significant bank debt maturities.  We believe that the large dollar volume of loans that need to be refinanced will present attractive opportunities to invest capital in a manner consistent with our stated objectives.



•

Middle market companies continue to face difficulties in accessing the capital markets.  We believe opportunities to serve the middle market will continue to exist. While many middle market companies were formerly able to raise funds by issuing high-yield bonds, we believe this approach to financing has become more difficult as institutional investors have sought to invest in larger, more liquid offerings.



•

Increased regulatory scrutiny of banks has reduced middle market lending.  We believe that many traditional bank lenders to middle market businesses have either exited or de-emphasized their service and product offerings in the middle market. These traditional lenders have instead focused on lending and providing other services to large corporate clients. We believe this has resulted in fewer key players and the reduced availability of debt capital to the companies we target.

•

Conservative loan to value.  As a result of the credit crisis, many lenders are requiring larger equity contributions from financial sponsors. Larger equity contributions create an enhanced margin of safety for lenders because leverage is a lower percentage of the implied enterprise value of the company.



•

Attractive pricing.  Reduced access to, and availability of, debt capital typically increases the interest rates, or pricing, of loans for middle market lenders. Recent primary debt transactions in this market often include upfront fees, original issue discount, prepayment protections and, in some cases, warrants to purchase common stock, all of which should enhance the profitability of new loans to lenders.

Operating and Regulatory Structure

          We are a closed-end, non-diversified management investment company that has elected to be regulated as a BDC under the 1940 Act and are required to maintain an asset coverage ratio, as defined in the 1940 Act, of at least 150.0%, which was reduced from 200% effective as of June 9, 2018 by approval of our stockholders. Changing the asset coverage ratio permits us to double our leverage, which may result in increased leverage risk and increased expenses. We include the assets and liabilities of our consolidated subsidiaries for purposes of satisfying the requirements under the 1940 Act. See "Regulation — Senior Securities" in the accompanying prospectus.

          We have elected to be treated, and intend to comply with the requirements to continue to qualify annually, as a RIC under Subchapter M of the Code. See "Material U.S. Federal Income Tax Considerations" in the accompanying prospectus. As a RIC, we generally will not be subject to corporate-level U.S. federal income taxes on any net ordinary income or capital gains that we timely distribute to our stockholders as dividends if it meets certain source-of-income, distribution and asset diversification requirements. We intend to distribute to our stockholders substantially all of our annual taxable income except that we may retain certain net capital gains for reinvestment.

Risks

          An investment in our securities involves risk, including the risk of leverage and the risk that our operating policies and strategies may change without prior notice to our stockholders or prior stockholder approval. See "Risk Factors" and the other information included in this prospectus supplement and the accompanying prospectus for a discussion of factors you should carefully consider before deciding to invest in our securities. The value of our assets, as well as the market price of our securities, will fluctuate. Our investments may be risky, and you may lose all or part of your investment. Investing in us involves other risks, including the following:

•

We may suffer credit losses;



•

We do not expect to replicate the Predecessor Entities' historical performance or the historical performance of other entities managed or supported by New Mountain Capital;



•

There is uncertainty as to the value of our portfolio investments because most of our investments are, and may continue to be, in private companies and recorded at fair value;



•

Our ability to achieve our investment objective depends on key investment personnel of the Investment Adviser. If the Investment Adviser were to lose any of its key investment personnel, our ability to achieve our investment objective could be significantly harmed;



•

The Investment Adviser has limited experience managing a BDC or a RIC, which could adversely affect our business;



•

We operate in a highly competitive market for investment opportunities and may not be able to compete effectively;

•

Our investments in securities rated below investment grade are speculative in nature and are subject to additional risk factors such as increased possibility of default, illiquidity of the security, and changes in value based on changes in interest rates;



•

Our business, results of operations and financial condition depend on our ability to manage future growth effectively;



•

We borrow money, which could magnify the potential for gain or loss on amounts invested in us and increase the risk of investing in us;



•

Changes in interest rates may affect our cost of capital and net investment income;



•

Regulations governing the operations of BDCs will affect our ability to raise additional equity capital as well as our ability to issue senior securities or borrow for investment purposes, any or all of which could have a negative effect on our investment objectives and strategies;



•

We may experience fluctuations in our annual and quarterly results due to the nature of our business;



•

Our board of directors may change our investment objective, operating policies and strategies without prior notice or stockholder approval, the effects of which may be adverse to your interests;



•

We will be subject to corporate-level U.S. federal income tax on all of our income if we are unable to maintain tax treatment as a RIC under Subchapter M of the Code, which would have a material adverse effect on our financial performance;



•

We cannot predict how tax reform legislation will affect us, our investments, or our stockholders, and any such legislation could adversely affect our business;



•

Recent legislation may allow us to incur additional leverage, which could increase the risk of investing in the Company;



•

Internal and external cyber threats, as well as other disasters, could impair our ability to conduct business effectively;



•

We may not be able to pay you distributions on our common stock, our distributions to you may not grow over time and a portion of our distributions to you may be a return of capital for U.S. federal income tax purposes;



•

Our investments in portfolio companies may be risky, and we could lose all or part of any of our investments;



•

The lack of liquidity in our investments may adversely affect our business;



•

Economic recessions, downturns or government spending cuts could impair our portfolio companies and harm our operating results;



•

The market price of our common stock may fluctuate significantly; and



•

Sales of substantial amounts of our common stock in the public market may have an adverse effect on the market price of our common stock.

Company Information

          Our administrative and executive offices are located at 787 Seventh Avenue, 48th Floor, New York, New York 10019, and our telephone number is (212) 720-0300. We maintain a website at www.newmountainfinance.com. Information contained on our website is not incorporated by reference into this prospectus supplement or the accompanying prospectus, and you should not

consider information contained on our website to be part of this prospectus supplement or the accompanying prospectus.

Presentation of Historical Financial Information and Market Data

Historical Financial Information

          Unless otherwise indicated, historical references contained in this prospectus supplement and the accompanying prospectus for periods prior to and as of December 31, 2013 in "Selected Financial and Other Data", "Selected Quarterly Data", "Management's Discussion and Analysis of Financial Condition and Results of Operations" and "Senior Securities" relate to NMF Holdings. The consolidated financial statements of New Mountain Finance Holdings, L.L.C., formerly known as New Mountain Guardian (Leveraged), L.L.C., and New Mountain Guardian Partners, L.P. are NMF Holdings' historical consolidated financial statements.

Market Data

          Statistical and market data used in this prospectus supplement and the accompanying prospectus has been obtained from governmental and independent industry sources and publications. We have not independently verified the data obtained from these sources, and we cannot assure you of the accuracy or completeness of the data. Forward-looking information obtained from these sources is subject to the same qualifications and the additional uncertainties regarding the other forward-looking statements contained in this prospectus supplement and accompanying prospectus. See "Cautionary Statement Regarding Forward-Looking Statements" in this prospectus supplement and the accompanying prospectus.

THE OFFERING

(1)

The Investment Adviser has agreed to bear all of the sales load in connection with this offering, which is not reflected in the above table and will not be subject to reimbursement by us. There is no guaranty that there will be any sales of our common stock pursuant to this prospectus supplement or the accompanying prospectus.

(2)

The offering expenses of this offering are estimated to be approximately $0.3 million.

(3)

If a participant elects by written notice to the plan administrator to have the plan administrator sell part or all of the shares held by the plan administrator in the participant's account and remit the proceeds to the participant, the plan administrator is authorized to deduct a $15.00 transaction fee plus a $0.10 per share brokerage commission from the proceeds. The expenses of the dividend reinvestment plan are included in "other expenses." The plan administrator's fees will be paid by us. There will be no brokerage charges or other charges to stockholders who participate in the plan.

(4)

The base management fee under the Investment Management Agreement is based on an annual rate of 1.75% of our average gross assets for the two most recent quarters, which equals our total assets on the Consolidated Statements of Assets and Liabilities, less (i) the borrowings under the SLF Credit Facility and (ii) cash and cash equivalents. We have not invested, and currently do not invest, in derivatives. To the extent we invest in derivatives in the future, we will use the actual value of the derivatives, as reported on our Consolidated Statements of Assets and Liabilities, for purposes of calculating our base management fee. Since our IPO, the base management fee calculation has deducted the borrowings under the SLF Credit Facility. The SLF Credit Facility had historically consisted of primarily lower yielding assets at higher advance rates. As part of an amendment to our existing credit facilities with Wells Fargo Bank, National Association, the SLF Credit Facility merged with the Predecessor Holdings Credit Facility and into the Holdings Credit Facility on December 18, 2014. Post credit facility merger and to be consistent with the methodology since our IPO, the Investment Adviser will continue to waive management fees on the leverage associated with those assets that share the same underlying yield characteristics with investments leveraged under the legacy SLF Credit Facility. The Investment Adviser cannot recoup management fees that the Investment Adviser has previously waived. The base management fee reflected in the table above is based on the three months ended September 30, 2018 and is calculated without deducting any management fees waived. The annual base management fee after deducting the management fee waiver

as a percentage of net assets would be 3.22% based on the three months ended September 30, 2018. See "Investment Management Agreement" in the accompanying prospectus.

(5)

Assumes that annual incentive fees earned by the Investment Adviser remain consistent with the gross incentive fees earned by the Investment Adviser during the three months ended September 30, 2018 and includes accrued capital gains incentive fee and calculated without deducting any incentive fees waived. For the three months ended September 30, 2018, no incentive fees were waived by the Investment Adviser. The Investment Adviser cannot recoup incentive fees that the Investment Adviser has previously waived. As of September 30, 2018, we did not have a capital gains incentive fee accrual. As we cannot predict whether we will meet the thresholds for incentive fees under the Investment Management Agreement, the incentive fees paid in subsequent periods, if any, may be substantially different than the fees incurred during the three months ended September 30, 2018. For more detailed information about the incentive fee calculations, see the "Investment Management Agreement" section of the accompanying prospectus.

(6)

We may borrow funds from time to time to make investments to the extent we determine that additional capital would allow us to take advantage of additional investment opportunities or if the economic situation is otherwise conducive to doing so. The costs associated with these borrowings are indirectly borne by our stockholders. As of September 30, 2018, we had $466.0 million, $135.0 million, $270.3 million, $335.0 million and $165.0 million of indebtedness outstanding under the Holdings Credit Facility, the NMFC Credit Facility, the Convertible Notes, the Unsecured Notes and the SBA-guaranteed debentures, respectively. For purposes of this calculation, we have assumed the September 30, 2018 amounts outstanding under the credit facilities, the Convertible Notes, the Unsecured Notes and the SBA-guaranteed debentures, and have computed interest expense using an assumed interest rate of 4.6% for the Holdings Credit Facility, 4.6% for the NMFC Credit Facility, 5.3% for the Convertible Notes, 5.2% for the Unsecured Notes and 3.3% for the SBA-guaranteed debentures, which were the rates payable as of September 30, 2018. See "Senior Securities" in this prospectus supplement and the accompanying prospectus.

(7)

"Other expenses" include our overhead expenses, including payments by us under the Administration Agreement based on the allocable portion of overhead and other expenses incurred by the Administrator in performing its obligations to us under the Administration Agreement. Pursuant to the Administration Agreement, the Administrator may, in its own discretion, submit to us for reimbursement some or all of the expenses that the Administrator has incurred on our behalf during any quarterly period. As a result, the amount of expenses for which we will have to reimburse the Administrator may fluctuate in future quarterly periods and there can be no assurance given as to when, or if, the Administrator may determine to limit the expenses that the Administrator submits to us for reimbursement in the future. However, it is expected that the Administrator will continue to support part of our expense burden in the near future and may decide to not calculate and charge through certain overhead related amounts as well as continue to cover some of the indirect costs. The Administrator cannot recoup any expenses that the Administrator has previously waived. This expense ratio is calculated without deducting any expenses waived or reimbursed by the Administrator. Assuming the expenses waived or reimbursed by the Administrator at September 30, 2018 of $0.3 million, the annual expense ratio after deducting the expenses waived or reimbursed by the Administrator as a percentage of net assets would be 0.91%. For the nine months ended September 30, 2018, we reimbursed the Administrator approximately $1.4 million for indirect administrative expenses, which represents approximately 0.18% of our net assets on an annulized basis. See "Administration Agreement" in the accompanying prospectus.

(8)

The holders of shares of our common stock indirectly bear the expenses of our investment in NMFC Senior Loan Program I, LLC ("SLP I"), NMFC Senior Loan Program II, LLC ("SLP II") and NMFC Senior Loan Program III, LLC ("SLP III"). No management fee is charged on our investment in SLP I in connection with the administrative services provided to SLP I. As SLP II and SLP III are structured as private joint ventures, no management fees are paid by SLP II or SLP III. Future expenses for SLP I, SLP II and SLP III may be substantially higher or lower because certain expenses may fluctuate over time.

(9)

The holders of shares of our common stock indirectly bear the cost associated with our annual expenses.

(1)

In applying the if-converted method, conversion is not assumed for purposes of computing diluted earnings per share if the effect would be anti-dilutive. For the year ended December 31, 2015, there was anti-dilution. For the nine months ended September 30, 2018 and the years ended December 31, 2017, December 31, 2016 and December 31, 2014, there was no anti-dilution. For the year ended December 31, 2013, due to reflecting earnings for the full year of operations of the

Predecessor Operating Company assuming 100.0% NMFC ownership of Predecessor Operating Company and assuming all of New Mountain Finance AIV Holdings Corporation's ("AIV Holdings") units in the Predecessor Operating Company were exchanged for public shares of NMFC during the year then ended, the earnings per share would be $1.79.

(2)

Distributions declared in the year ended December 31, 2014 include a $0.12 per share special dividend related to realized capital gains attributable to NMF Holdings' warrant investments in Learning Care Group (US), Inc. Distributions declared in the year ended December 31, 2013 include a $0.12 per share special dividend related to a distribution received attributable to NMF Holdings' investment in YP Equity Investors LLC.

(3)

On January 1, 2016, we adopted Accounting Standards Update No. 2015-03, Interest — Imputation of Interest Subtopic 835-30 — Simplifying the Presentation of Debt Issuance Costs ("ASU 2015-03"). Upon adoption, we revised our presentation of deferred financing costs from an asset to a liability, which is a direct deduction to our debt on the Consolidated Statements of Assets and Liabilities. In addition, as of December 31, 2015 and December 31, 2014, we retrospectively revised our presentation of $14.0 million and $14.1 million, respectively, of deferred financing costs that were previously presented as an asset, which resulted in a decrease to total assets and total liabilities as of December 31, 2015 and December 31, 2014. For the years ended December 31, 2013 and December 31, 2012, NMFC was a holding company with no direct operations of its own and its sole asset was its ownership in the Predecessor Operating Company and, as such, ASU 2015-03 did not apply to NMFC.

(4)

Total return is calculated assuming a purchase of common stock at the opening of the first day of the period and a sale on the closing of the last business day of the respective period ends. Dividends and distributions, if any, are assumed for purposes of this calculation, to be reinvested at prices obtained under our dividend reinvestment plan.

(5)

Total return is calculated assuming a purchase at net asset value on the opening of the first day of the period and a sale at net asset value on the last day of the period. Dividends and distributions, if any, are assumed for purposes of this calculation, to be reinvested at the net asset value on the last day of the respective quarter.

(6)

For the year ended December 31, 2014, amounts include our investment activity and the investment activity of the Predecessor Operating Company.

(7)

The weighted average YTM at Cost calculation assumes that all investments, including secured collateralized agreements, not on non-accrual are purchased at the adjusted cost on the respective period ends and held until their respective maturities with no prepayments or losses and exited at par at maturity. Adjusted cost reflects the cost for post-IPO investments in accordance with GAAP and a stepped up cost basis of pre-IPO investments (assuming a step-up to fair market value occurred on the IPO date).

(8)

The weighted average YTM at Cost for Investments calculation assumes that all investments, including secured collateralized agreements, are purchased at the adjusted cost on the respective period ends and held until their respective maturities with no prepayments or losses and exited at par at maturity. Adjusted cost reflects the cost for post-IPO investments in accordance with GAAP and a stepped up cost basis of pre-IPO investments (assuming a step-up to fair market value occurred on the IPO date).

(1)

Distributions declared in the year ended December 31, 2013 include a $0.12 per unit special dividend related to a distribution received attributable to NMF Holdings' investment in YP Equity Investors LLC. Actual cash payments on the distributions declared to AIV Holdings only, for the quarter ended March 31, 2013 was made on April 5, 2013.

(2)

Total return is calculated assuming a purchase at net asset value on the opening of the first day of the year and a sale at net asset value on the last day of the respective period ends. Dividends and distributions, if any, are assumed for purposes of this calculation, to be reinvested at the net asset value on the last day of the respective quarter. Dividends and distributions, if any, are assumed for purposes of this calculation, to be reinvested at the net asset value on the last day of the respective quarter.

(3)

The weighted average YTM at Cost calculation assumes that all investments not on non-accrual are purchased at the adjusted cost on the respective period ends and held until their respective maturities with no prepayments or losses and exited at par at maturity. Adjusted cost reflects the GAAP cost for post-IPO investments and a stepped up cost basis of pre-IPO investments (assuming a step-up to fair market value occurred on the IPO date).

(4)

The weighted average YTM calculation assumes that all investments not on non-accrual are purchased at fair value on the respective period ends and held until their respective maturities with no prepayments or losses and exited at par at maturity. The weighted average YTM was not calculated subsequent to December 31, 2013.

(5)

The weighted average YTM at Cost for Investments calculation assumes that all investments, including secured collateralized agreements, are purchased at the adjusted cost on the respective period ends and held until their respective maturities with no prepayments or losses and exited at par at maturity. Adjusted cost reflects the cost for post-IPO investments in accordance with GAAP and a stepped up cost basis of pre-IPO investments (assuming a step-up to fair market value occurred on the IPO date).

(1)

Includes securities purchased under collateralized agreements to resell, benefit (provision) for taxes and the accretive effect of common stock issuances per share, if applicable.