Exxon Mobil Corp. hired a company outsider to lead its finances, a move that comes as the energy giant faces investor pressure to take more steps to transition away from fossil fuels.

Irving, Texas-based Exxon on Monday said Kathryn Mikells would take over as chief financial officer on Aug. 9. Ms. Mikells, who previously held CFO positions at printer maker Xerox Holdings Corp. , security systems company ADT Inc. and United Airlines Holdings Inc., most recently served as finance chief of British spirits maker Diageo PLC. Ms. Mikells worked in that position from November 2015 to June 2021 and recently returned to the U.S. to be closer to her family.

At Exxon, Ms. Mikells succeeds Andrew Swiger, who became the company’s principal financial officer in 2013 and intends to retire, effective Sept. 1. Mr. Swiger, a 43-year company veteran, joined Exxon as an operations engineer in Morgan City, La., and took on various positions as he rose through the ranks. He is 64 years old. Exxon’s mandatory retirement age is 65.

Kathryn Mikells

Kathryn Mikells

Photo: Diageo

The appointment of an external candidate like Ms. Mikells, who has never worked at Exxon, is a rare choice for the company. The oil company has historically preferred to promote from within its own ranks, especially for positions on the management committee, which employees internally refer to as the “God pod.”

Exxon has faced pressure from investors and other stakeholders over the past year, culminating in the defeat of three of the company’s nominees to its board at its annual shareholder meeting in May. Engine No. 1, a hedge fund created last year, successfully placed three directors on Exxon’s board despite owning just 0.02% of its stock.

Engine No. 1 criticized Exxon’s corporate culture as being too insular, and said the company needed outside perspectives to help it navigate a continuing industry transition to lower carbon energy sources. The hedge fund also attacked Exxon’s financial planning and capital allocation, saying the company had weakened its balance sheet with ill-timed investments over the last decade.

At Diageo, Ms. Mikells focused on reducing costs and reshaping the company’s portfolio, including by buying brands like Casamigos tequila in 2017 and Aviation American Gin last year while divesting the company of some other brands. She also led investments to increase Diageo’s stake in its Chinese baijiu liquor business and expand operations in Africa and Scotland.

During Ms. Mikells’s tenure, Diageo turned to a suite of tools known as predictive analytics that gather and crunch large amounts of data to identify trends and forecast customer demand and commodity prices. She also made use of zero-based budgeting, a technique that forces finance managers to plan each year’s budget as if starting from scratch.

Exxon didn’t respond to a request for additional comment.

—Kristin Broughton contributed to this article.

Write to Nina Trentmann at nina.trentmann@wsj.com and Christopher M. Matthews at christopher.matthews@wsj.com