UPDATE 2-Marathon Petroleum names insider Maryann Mannen as CEO

(Adds background, details on CEO-elect's prior positions, CEO comment throughout)

May 13 (Reuters) - Marathon Petroleum, the largest U.S. oil refiner, on Monday named President Maryann Mannen to succeed Michael Hennigan as chief executive officer effective Aug. 1.

Hennigan, 64, successfully underwent treatment for throat cancer during his nearly four years at the helm of the largest U.S. crude oil refiner. He becomes executive chairman on stepping down as CEO, the company said.

Three years ago, Marathon closed the $21 billion sale of its Speedway gasoline retail operations to 7-Eleven, a unit of Seven & i Holdings, following a long pressure campaign by investor Elliott Investment Management.

Mannen "has demonstrated a keen understanding of our business, clear judgment and a strong financial expertise that will serve shareholders well," Hennigan said in a statement.

Mannen joined the Findlay, Ohio-based refiner from TechnipFMC as finance chief in 2021 and was promoted to president on Jan. 1.

As CEO, she will run a refiner that processes 3 million barrels per day of crude oil across 13 refineries.

Hennigan will replace independent chairman John Surma, 69. (Reporting by Seher Dareen in Bengaluru; Editing by Alan Barona and Bill Berkrot)