Marathon Petroleum (No. 16 on the Fortune 500) announced on Monday that Maryann T. Mannen will succeed Michael J. Hennigan as CEO, and join the board of directors, effective Aug. 1. Mannen, the company's president since Jan. 1, will become the first female CEO at Marathon, where she's served as EVP and CFO since 2021, when she became the firm's first female CFO.

Before joining the Findlay, Ohio-based oil refiner, Mannen served as EVP and CFO of TechnipFMC (a successor to FMC Technologies, Inc.), a global engineering services and energy technology company. She also previously was EVP and CFO of FMC Technologies, which she joined in 1986.

"I am honored by the trust placed in me to lead this great company,” Mannen said in a statement. Her priorities at Marathon will continue a focus on safety, and environmental and operational performance.

As chief executive, Mannen's CFO experience should prove helpful. “In most companies today, the CFO is viewed as the clear No. 2 to the CEO,” Scott W. Simmons, co-managing partner of executive search firm Crist Kolder Associates, told Fortune. "This expansion of responsibility, coupled with the unique vantage point from which a CFO operates, better positions CFOs as CEO successors."

Based on data from 674 Fortune 500 and S&P 500 companies, last year 8.4% percent of CEO roles were filled by CFOs; in 2013, that figure was 5.8%, according to the firm’s most recent volatility report. Fortune recently reported that women CEOs were now running more than 10% of the Fortune 500. “I absolutely believe the increase in the number of women CEOs will continue, as we have a long way to go on the path to correction,” Simmons added.

Hennigan, who became Marathon’s CEO in March 2020, is transitioning to executive chairman as the current chair, John Surma, in the role since April 2020, moves to lead director.

“The board unanimously supports and looks forward to Maryann's leadership as our next CEO,” Surma said in a statement. “It's an exciting time in the energy sector, and she will take the helm of a company with exceptional employees and an outstanding set of assets."

Hennigan said in a statement that Mannen is the “right leader for our company” based on her contributions to MPC and “proven track record as an energy sector veteran of 38 years.” He added that her “keen understanding of our business, clear judgment and a strong financial expertise” will serve shareholders well.

Marathon operates the nation's largest refining system, with 3 million barrels per calendar day of crude oil refining capacity across 13 refineries. In 2021, Marathon closed the $21 billion sale of Speedway to 7-Eleven.

This story was originally featured on Fortune.com