Page 1

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington , D.C. 20549 Form 10 - K ANNUAL REPORT PURSUANT TO SECTION 13 OR 15 ( d ) OF THE SECURITIES EXCHANGE ACT OF 1934 For the fiscal year ended December 31 , 2020 □ TRANSITION REPORT PURSUANT TO SECTION 13 OR 15 ( d ) OF THE SECURITIES EXCHANGE ACT OF 1934 For the transition period from Commission file number 001-35714 to MPLX LP ( Exact name of registrant as specified in its charter ) Delaware ( State or other jurisdiction of incorporation or organization ) Title of each class Common Units Representing Limited Partnership Interests 200 E. Hardin Street , Findlay , OH 45840-3229 ( Address of principal executive offices ) ( Zip code ) ( 419 ) 421-2414 ( Registrant's telephone number , including area code ) Securities registered pursuant to Section 12 ( b ) of the Act : Trading Symbol ( s ) MPLX ( I.R.S. Employer Identification No. ) 27-0005456 Name of each exchange on which registered New York Stock Exchange No " No X Securities registered pursuant to Section 12 ( g ) of the Act : None Indicate by check mark if the registrant is a well - known seasoned issuer , as defined in Rule 405 of the Securities Act . Yes x Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15 ( d ) of the Act . Yes Indicate by check mark whether the registrant ( 1 ) has filed all reports required to be filed by Section 13 or 15 ( d ) of the Securities Exchange Act of 1934 during the preceding 12 months ( or for such shorter period that the registrant was required to file such reports ) , and ( 2 ) has been subject to such filing requirements for the past 90 days . Yes x No " Indicate by check mark whether the registrant has submitted electronically every Interactive Data File required to be submitted pursuant to Rule 405 of Regulation ( §232.405 of this chapter ) during the preceding 12 months ( or for such shorter period that the registrant was required to submit files ) . Yes x No " Indicate by check mark whether the registrant is a large accelerated filer , an accelerated filer , a non - accelerated filer , a smaller reporting company or an emerging growth company . See the definitions of " large accelerated filer , " " accelerated filer , " " smaller reporting company " and " emerging growth company " in Rule 12b - 2 of the Exchange Act . Large Accelerated Filer x Accelerated filer Non - accelerated filer " Smaller reporting company Emerging growth company If an emerging growth company , indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13 ( a ) of the Exchange Act . Indicate by check mark whether the registrant has filed a report on and attestation to its management's assessment of the effectiveness of its internal control over financial reporting under Section 404 ( b ) of the Sarbanes - Oxley Act ( 15 U.S.C. 7262 ( b ) ) by the registered public accounting firm that prepared or issued its audit report . X Indicate by check mark whether the registrant is a shell company ( as defined in Rule 12b - 2 of the Exchange Act ) . Yes No x The aggregate market value of common units held by non - affiliates as of June 30 , 2020 was approximately $ 6.8 billion . This amount is based on the closing price of the registrant's common units on the New York Stock Exchange on June 30 , 2020. Common units held by executive officers and directors of the registrant and its affiliates are not included in the computation . The registrant , solely for the purpose of this required presentation , has deemed its directors and executive officers and those of its affiliates to be affiliates . MPLX LP had 1,037,169,607 common units outstanding at February 12 , 2021 . DOCUMENTS INCORPORATED BY REFERENCE : None