After the antitrust case against Jersey Standard in 1911 and subsequent breakup of its holdings, Ohio Oil once again became an independent company.

After the acquisition, the USX Corporation was created to act as the parent of U.S. Steel and Marathon Oil, which operated as divisions.

As of December 31, 2020, the company had 972 million barrels of oil equivalent (5.95×109 GJ) of estimated proven reserves, of which 86% was in the United States and 14% was in Equatorial Guinea.

[1] In 2020, the company sold 383 thousand barrels of oil equivalent (2,340,000 GJ) per day, of which 26% was from the Eagle Ford Group, 27% was from the Bakken formation, 17% was from Oklahoma, 7% was from the Northern Delaware Basin, 2% was from other U.S. sources, and 20% was from Equatorial Guinea.

[19] In September 2013, Marathon sold a 10% stake in an oil and gas field offshore Angola for $590 million to Sonangol Group.

[24][25] In December 2022, the company acquired assets in the Eagle Ford from Ensign Natural Resources for $3.0 billion in cash.

[28][29] In July 2024, Marathon agreed to a $241.5 million settlement with the US Department of Justice and the Environmental Protection Agency to resolve allegations of failing to obtain required permits at dozens of the company's oil and gas facilities on the Fort Berthold Indian Reservation in North Dakota and releasing thousands of tons of illegal air pollution as a result.

[30] Since 2003, Marathon Oil and its partners Noble Energy and AMPCO have invested in the Bioko Island Malaria Control Project (BIMCP) in Equatorial Guinea.

[30][32] The company was investigated for payments made to Teodoro Obiang Nguema Mbasogo, the president of Equatorial Guinea.