Prospectus summary

This summary highlights selected information contained elsewhere in this prospectus. You should carefully read the entire prospectus, including “Risk Factors” and the historical and unaudited pro forma combined financial statements and related notes included elsewhere in this prospectus before making an investment decision. Unless otherwise indicated, the information in this prospectus assumes that the underwriters do not exercise their option to purchase additional common units. You should read “Risk Factors” beginning on page 23 for more information about important factors that you should consider before purchasing our common units.

Unless the context otherwise requires, references in this prospectus to “MPLX LP,” “our partnership,” “we,” “our,” “us,” or like terms, when used in a historical context, refer to the assets that MPC is contributing to us in connection with this offering. These assets include a 100.0% interest in MPLX Terminal and Storage LLC, which owns our butane cavern, and our 51.0% general partner interest in MPLX Pipe Line Holdings LP, the 100.0% owner of Marathon Pipe Line LLC and Ohio River Pipe Line LLC, which are the subsidiaries of MPC that own our pipeline systems and associated crude oil and product storage assets. When used in the present tense or prospectively, these terms refer to MPLX LP and its subsidiaries, including Pipe Line Holdings. References to “our general partner” refer to MPLX GP LLC. References to “MPC” refer collectively to Marathon Petroleum Corporation and its subsidiaries, other than us, our subsidiaries and our general partner. References to “Pipe Line Holdings” refer to MPLX Pipe Line Holdings LP. References to “MPL” and “ORPL” refer to Marathon Pipe Line LLC and Ohio River Pipe Line LLC, respectively. While we will only own a 51.0% indirect interest in MPL and ORPL through Pipe Line Holdings, we refer to the assets owned by MPL and ORPL as our assets. In addition, unless otherwise specifically noted, financial results and operating data are shown on a 100.0% basis and are not adjusted to reflect MPC’s 49.0% limited partner interest in Pipe Line Holdings. We have provided definitions for some of the terms we use to describe our business and industry and other terms used in this prospectus in the “Glossary of Terms” beginning on page B-1 of this prospectus.

MPLX LP

OVERVIEW

We are a fee-based, growth-oriented limited partnership recently formed by Marathon Petroleum Corporation to own, operate, develop and acquire crude oil, refined product and other hydrocarbon-based product pipelines and other midstream assets. Our assets primarily consist of a 51.0% indirect interest in a network of common carrier crude oil and product pipeline systems and associated storage assets in the Midwest and Gulf Coast regions of the United States. We believe our network of petroleum pipelines is one of the largest in the United States, based on total annual volumes delivered. MPC has retained a 49.0% interest in our network of pipeline systems and associated storage assets; however, we control and operate these assets and refer to them throughout this prospectus as our pipeline systems and our assets. We also own a 100.0% interest in a butane cavern in Neal, West Virginia with approximately 1.0 million barrels of storage capacity. Our assets are integral to the success of MPC’s operations.

We generate revenue primarily by charging tariffs for transporting crude oil, refined products and other hydrocarbon-based products through our pipelines and at our barge dock and fees for storing crude oil and products at our storage facilities. We are also the operator of additional crude oil and product pipelines owned by MPC and third parties for which we are paid operating fees. We do not take ownership of the crude oil or products that we transport and store for our customers, and we do not engage in the trading of any commodities.

MPC historically has been the source of the substantial majority of our revenues. In connection with this offering, we have entered into multiple storage services agreements and, at the closing of this offering, we will enter into multiple transportation services agreements with MPC. These agreements, which we refer to in this prospectus as our transportation and storage services agreements, are long-term, fee-based agreements with minimum volume commitments under which MPC will continue to be the source of the substantial majority of our revenues for the foreseeable future. We believe these transportation and storage services agreements will promote stable and predictable cash flows.

MPC has stated that it intends for us to be the primary growth vehicle for its midstream business. Following the completion of this offering, MPC will continue to own a substantial portfolio of other midstream assets, including the remaining 49.0% indirect interest in our network of pipeline systems, our barge dock and our storage tanks. MPC will also retain a significant interest in us through its ownership of our general partner, a 75.1% limited partner interest in us and all of our incentive distribution rights. Given MPC’s significant ownership interest in us following this offering and its stated intent to use us to grow its midstream business, we believe MPC will offer us the opportunity to purchase additional midstream assets that it owns, including additional interests in Pipe Line Holdings. We also may pursue acquisitions jointly with MPC. MPC is under no obligation, however, to offer to sell us additional assets or to pursue acquisitions jointly with us, and we are under no obligation to buy any such additional assets or pursue any such joint acquisitions. We also intend to grow our business by constructing new assets, increasing the utilization of, and revenue generated by, our existing assets and acquiring assets from third parties.

OUR ASSETS AND OPERATIONS

Our primary assets consist of:

As the sole general partner of Pipe Line Holdings, we will control all aspects of the management of Pipe Line Holdings, including its cash distribution policy. The only outstanding partnership interests in Pipe Line Holdings will be our 51.0% general partner interest and the 49.0% limited partner interest retained by MPC.

The following table sets forth certain information regarding our crude oil pipeline systems, each of which will have, as of the closing of this offering, an associated transportation services agreement with MPC:

Crude Oil Pipeline Systems

The following table sets forth certain information regarding our product pipeline systems, each of which will have, as of the closing of this offering, an associated transportation services agreement with MPC (other than our Louisville Airport products system, which currently transports only third-party volumes):

Product Pipeline Systems

The following table sets forth certain information regarding our other midstream assets, each of which currently has, or will have, as of the closing of this offering, an associated transportation services agreement or storage services agreement with MPC:

Other Midstream Assets

For the year ended December 31, 2011 and the six months ended June 30, 2012, on a pro forma basis, we had revenues and other income of approximately $352.0 million and $194.2 million, Adjusted EBITDA of approximately $123.8 million and $70.6 million and net income of approximately $94.3 million and $55.6 million, respectively. Excluding revenues attributable to volumes shipped by MPC under joint tariffs with third parties that were treated as third party revenues for accounting purposes, MPC accounted for approximately 80% and 81% of our pro forma revenues and other income for those periods, respectively. After excluding the 49.0% interest in Pipe Line Holdings retained by MPC, pro forma Adjusted EBITDA attributable to MPLX LP was approximately $63.0 million and $35.9 million and pro forma net income attributable to MPLX LP was approximately $48.0 million and $28.2 million for those same periods, respectively. Please read “Selected Historical and Pro Forma Financial and Operating Data” for the definition of the term Adjusted EBITDA and a reconciliation of Adjusted EBITDA to our most directly comparable financial measures, calculated and presented in accordance with GAAP.

BUSINESS STRATEGIES

Our primary business objectives are to generate stable cash flows and increase our quarterly cash distribution per unit over time. We intend to accomplish these objectives by executing the following strategies:

COMPETITIVE STRENGTHS

We believe we are well positioned to execute our business strategies based on the following competitive strengths:

OUR RELATIONSHIP WITH MARATHON PETROLEUM CORPORATION

One of our principal strengths is our relationship with MPC, which we believe to be the fifth-largest petroleum products refiner in the United States and the largest petroleum products refiner in the Midwest region of the United States based on crude oil refining capacity. MPC owns and operates six refineries and associated midstream transportation and logistics assets in PADD II and PADD III, which consist of states in the Midwest and Gulf Coast regions of the United States, along with an extensive wholesale and retail refined product marketing operation that serves markets primarily in the Midwest and Southeast regions of the United States. MPC markets refined products under the Marathon brand through an extensive network of jobber- and dealer-owned retail locations, and under the Speedway brand through its wholly owned subsidiary, Speedway LLC, which operates what we believe to be the nation’s fourth-largest chain of company-owned and operated retail gasoline and convenience stores. For the year ended

December 31, 2011 and the six months ended June 30, 2012, MPC had consolidated revenues of approximately $78.6 billion and $40.5 billion, respectively. Marathon Petroleum Corporation’s common stock trades on the New York Stock Exchange (“NYSE”) under the symbol “MPC.”

MPC’s operations necessitate large-scale movements of crude oil and feedstocks to and among its refineries, as well as large-scale movements of refined products from its refineries to various markets. To this end, MPC has an extensive, integrated network of midstream assets. Following the completion of this offering, MPC will continue to own or lease a substantial portfolio of midstream assets, including:

MPC will retain a significant interest in us through its ownership of our general partner, a 75.1% limited partner interest in us and all of our incentive distribution rights. We believe MPC will promote and support the successful execution of our business strategies given its significant ownership in us following this offering and its stated intention to use us to grow its midstream business. As a result, we believe MPC will offer us the opportunity to purchase additional assets from it, including additional interests in Pipe Line Holdings. We also may pursue acquisitions jointly with MPC. However, MPC is under no obligation to offer to sell us additional assets or to pursue acquisitions jointly with us, and we are under no obligation to buy any such additional assets or pursue any such joint acquisitions.

In connection with this offering, we have entered into multiple storage services agreements and, at the closing of this offering, we will enter into multiple transportation services agreements with MPC. Under these long-term, fee-based transportation and storage services agreements, we will provide transportation and storage services to MPC, and MPC will commit to provide us with minimum quarterly throughput and storage volumes of crude oil and products and minimum storage volumes of butane.

In addition to our transportation and storage services agreements with MPC, we will enter into an omnibus agreement with MPC in connection with this offering. The omnibus agreement will address our payment of an annual amount to MPC for the provision of certain general and administrative services and MPC’s indemnification of us for certain matters, including environmental, title and tax matters. We have also entered into two management services agreements and two employee services agreements with MPC in connection with this offering. Under our management services agreements with MPC, MPC has agreed to pay us fixed monthly fees for providing certain management services to MPC with respect to certain of MPC’s retained pipeline assets. Under our employee services agreements with MPC, we have agreed to reimburse MPC for the provision of certain operational and management services to us in support of our assets. While not the result of arm’s-length negotiations, we believe the terms of all of our initial agreements with MPC are, and specifically intend the rates to be, generally no less favorable to

either party than those that could have been negotiated with unaffiliated parties with respect to similar services. Please read “Certain Relationships and Related Party Transactions—Agreements Governing the Transactions.”

While our relationship with MPC and its subsidiaries is a significant strength, it is also a source of potential conflicts. Please read “Conflicts of Interest and Duties” and “Risk Factors—Risks Inherent in an Investment in Us—Our general partner and its affiliates, including MPC, have conflicts of interest with us and limited duties to us and our unitholders, and they may favor their own interests to our detriment and that of our unitholders. Additionally, we have no control over MPC’s business decisions and operations, and MPC is under no obligation to adopt a business strategy that favors us.”

OUR TRANSPORTATION AND STORAGE SERVICES AGREEMENTS WITH MPC

Our assets are strategically located within, and integral to, MPC’s operations. In connection with this offering, we have entered into multiple long-term, fee-based storage services agreements and, at the closing of this offering, we will enter into multiple long-term, fee-based transportation services agreements with MPC. These agreements include the following:

Under the transportation services agreements for our pipeline systems and barge dock, we may annually adjust the tariff rates paid by MPC. Each of the transportation services agreements for our crude oil and product pipeline systems (other than our Wood River to Patoka crude system) will automatically renew for up to two additional five-year terms unless terminated by either party no later than six months prior to the end of the term. The transportation services agreement for our Wood River to Patoka crude system and our Wood River barge dock will automatically renew for up to four additional two-year terms unless terminated by either party no later than six months prior to the end of the term. Our butane cavern storage services agreement will not automatically renew after the end of its 10-year term. Each of the storage services agreements for our tank farms will automatically renew for additional one-year terms unless terminated by either party no later than six months prior to the end of the term.

For the year ended December 31, 2011 and the six months ended June 30, 2012, on a pro forma basis, approximately 71% and 75% of our total revenues and other income, respectively, was attributable to MPC’s minimum volume commitments under these agreements.

For additional information about our transportation and storage services agreements with MPC, please read “Management’s Discussion and Analysis of Financial Condition and Results of Operations—How We Generate Revenue” and “Business—Our Transportation and Storage Services Agreements with MPC.”

OUR EMERGING GROWTH COMPANY STATUS

As a company with less than $1.0 billion in revenue during its last fiscal year, we qualify as an “emerging growth company” as defined in the Jumpstart Our Business Startups Act of 2012 (the “JOBS Act”), and we are eligible to take advantage of certain exemptions from various reporting requirements that are applicable to other public companies that are not “emerging growth companies” including, but not limited to, not being required to comply with the auditor attestation requirements of Section 404 of the Sarbanes-Oxley Act of 2002 and reduced disclosure obligations regarding executive compensation in our periodic and annual reports. We intend to take advantage of some of the reduced disclosure obligations regarding executive compensation.

In addition, Section 107 of the JOBS Act also provides that an emerging growth company can take advantage of the extended transition period provided in the Securities Act for complying with new or revised accounting standards. In other words, an emerging growth company can delay the adoption of certain accounting standards until those standards would otherwise apply to private companies. We will “opt out” of such extended transition period and will be required to comply with new or revised accounting standards on the relevant dates on which adoption of such standards is required for non-emerging growth companies. The JOBS Act provides that any decision to opt out of the extended transition period for complying with new or revised accounting standards is irrevocable.

We could remain an emerging growth company for up to five years, or until the earliest of (i) the last day of the first fiscal year in which our annual gross revenues exceed $1.0 billion, (ii) the date that we become a “large accelerated filer” as defined in Rule 12b-2 under the Securities Exchange Act of 1934, as amended, or the Exchange Act, which would occur if the market value of our common units that are held by non-affiliates exceeds $700.0 million as of the last business day of our most recently completed second fiscal quarter, and (iii) the date on which we have issued more than $1.0 billion in non-convertible debt during the preceding three-year period.

RISK FACTORS

An investment in our common units involves risks associated with our business, our partnership structure and the tax characteristics of our common units. You should carefully consider the risks described in “Risk Factors” and the other information in this prospectus before investing in our common units.

THE TRANSACTIONS

We were formed in March 2012 by MPC to own, operate, develop and acquire crude oil, refined products and other hydrocarbon-based product pipelines and other midstream assets.

Each of the following transactions have occurred or will occur in connection with this offering:

ORGANIZATIONAL STRUCTURE AFTER THE TRANSACTIONS

After giving effect to the transactions described above, our units will be held as follows:

The following simplified diagram depicts our organizational structure after giving effect to the transactions described above.

MANAGEMENT OF MPLX LP

We are managed and operated by the board of directors and executive officers of MPLX GP LLC, our general partner. MPC is the sole owner of our general partner and has the right to appoint the entire board of directors of our general partner, including the independent directors appointed in accordance with the listing standards of the NYSE. Unlike shareholders in a publicly traded corporation, our unitholders will not be entitled to elect our general partner or the board of directors of our general partner. Many of the executive officers and directors of our general partner also currently serve as executive officers and directors of MPC. For more information about the directors and executive officers of our general partner, please read “Management—Directors and Executive Officers of MPLX GP LLC.”

In order to maintain operational flexibility, our operations will be conducted through, and our operating assets will be owned by, various operating subsidiaries. However, neither we nor our subsidiaries have any employees. Our general partner has the sole responsibility for providing the personnel necessary to conduct our operations, whether through directly hiring employees or by obtaining the services of personnel employed by MPC or others. All of the personnel that will conduct our business immediately following the closing of this offering will be employed or contracted by our general partner and its affiliates, including MPC, but we sometimes refer to these individuals in this prospectus as our employees.

PRINCIPAL EXECUTIVE OFFICES AND INTERNET ADDRESS

Our principal executive offices are located at 200 E. Hardin Street, Findlay, Ohio 45840, and our telephone number is (419) 672-6500. Following the completion of this offering, our website will be located at www.mplx.com. We expect to make our periodic reports and other information filed with or furnished to the Securities and Exchange Commission (“SEC”) available, free of charge, through our website, as soon as reasonably practicable after those reports and other information are electronically filed with or furnished to the SEC. Information on our website or any other website is not incorporated by reference into this prospectus and does not constitute a part of this prospectus.

SUMMARY OF CONFLICTS OF INTEREST AND DUTIES

Under our partnership agreement, our general partner has a duty to manage us in a manner it believes is not adverse to the best interests of our partnership. However, because our general partner is a wholly owned subsidiary of MPC, the officers and directors of our general partner have a duty to manage the business of our general partner in a manner that is not adverse to the best interests of MPC. As a result of this relationship, conflicts of interest may arise in the future between us and our unitholders, on the one hand, and our general partner and its affiliates, including MPC, on the other hand. For example, our general partner will be entitled to make determinations that affect the amount of cash distributions we make to the holders of common units, which in turn has an effect on whether our general partner receives incentive cash distributions. In addition, our general partner may determine to manage our business in a way that directly benefits MPC’s refining or marketing businesses, whether by causing us not to seek higher tariff rates or not to connect our pipelines with those of other third parties or otherwise, rather than indirectly benefitting MPC solely through its ownership interests in us. For a more detailed description of the conflicts of interest and fiduciary duties of our general partner, please read “Conflicts of Interest and Duties.”

Delaware law provides that Delaware limited partnerships may, in their partnership agreements, expand, restrict or eliminate the fiduciary duties owed by the general partner to limited partners and the partnership. Our partnership agreement contains various provisions replacing the fiduciary duties that

would otherwise be owed by our general partner with contractual standards governing the duties of the general partner and contractual methods of resolving conflicts of interest. The effect of these provisions is to restrict the remedies available to unitholders for actions that might otherwise constitute breaches of our general partner’s fiduciary duties. Our partnership agreement also provides that affiliates of our general partner, including MPC and its other subsidiaries, are not restricted from competing with us, and neither our general partner nor its affiliates have any obligation to present business opportunities to us. By purchasing a common unit, the purchaser agrees to be bound by the terms of our partnership agreement, and pursuant to the terms of our partnership agreement each holder of common units consents to various actions and potential conflicts of interest contemplated in our partnership agreement that might otherwise be considered a breach of fiduciary or other duties under Delaware law. Please read “Conflicts of Interest and Duties—Duties of the General Partner” for a description of the fiduciary duties imposed on our general partner by Delaware law, the replacement of those duties with contractual standards under our partnership agreement and certain legal rights and remedies available to holders of our common units and subordinated units. For a description of our other relationships with our affiliates, please read “Certain Relationships and Related Party Transactions.”

The offering

Summary historical and pro forma financial and operating data

The following table shows summary historical combined financial and operating data of MPLX LP Predecessor, our Predecessor for accounting purposes (our “Predecessor”), as of the date and for the periods indicated and summary pro forma combined financial and operating data of MPLX LP as of the date and for the periods indicated. Our Predecessor consists of a 100.0% interest in all of the assets and operations of MPL and ORPL that MPC will contribute to us at the closing of this offering, as well as minority undivided interests in two crude oil pipeline systems, which we refer to as the joint interest assets, that will not be contributed to us. In connection with the closing of this offering, MPC will transfer the joint interest assets from our Predecessor to other MPC subsidiaries and then contribute to us a 51.0% indirect ownership interest in Pipe Line Holdings, which owns our Predecessor’s assets and operations (other than the joint interest assets), and a 100.0% indirect ownership in our butane cavern. However, as required by United States generally accepted accounting principles (“GAAP”), we will continue to consolidate 100.0% of the assets and operations of Pipe Line Holdings in our financial statements. In addition, we will record the contribution at historical cost, as it will be considered a reorganization of entities under common control.

The summary historical combined financial and operating data of our Predecessor as of and for the years ended December 31, 2009, 2010 and 2011 are derived from the audited combined financial data of our Predecessor appearing elsewhere in this prospectus. The summary historical interim combined financial data of our Predecessor as of and for the six months ended June 30, 2011 and 2012 are derived from the unaudited interim combined financial statements of our Predecessor appearing elsewhere in this prospectus. The following table should be read together with, and is qualified in its entirety by reference to, the historical audited and unaudited interim combined financial statements and the accompanying notes included elsewhere in this prospectus. The following table should also be read together with “Management’s Discussion and Analysis of Financial Condition and Results of Operations.”

The summary pro forma combined financial data presented in the following table for the year ended December 31, 2011 and as of and for the six months ended June 30, 2012 are derived from the unaudited pro forma combined financial data included elsewhere in this prospectus. The pro forma balance sheet assumes that the offering and the related transactions occurred as of June 30, 2012, and the pro forma statements of income for the year ended December 31, 2011 and the six months ended June 30, 2012 assume that the offering and the related transactions occurred as of January 1, 2011. These transactions primarily include, and the pro forma financial data give effect to, the following:

The pro forma financial data does not give effect to an estimated $3.4 million in incremental general and administrative expenses that we expect to incur annually as a result of being a separate publicly-traded partnership.