OKLAHOMA CITY — Oklahoma Gov. Kevin Stitt appointed Alan Armstrong on Tuesday to fill the U.S. Senate seat vacated by Markwayne Mullin through the end of the year. Armstrong, executive chairman of Williams Companies, has never served in elected office.

The appointment comes after Mullin was confirmed on Monday to serve as Secretary of Homeland Security. Armstrong will serve the final nine months of Mullin's Senate term, which was up for election in November.

Under Oklahoma law, a person appointed to fill a Senate vacancy must agree not to run for a full term in the following election. Stitt had pledged to choose a candidate described as a conservative voice to fill Mullin's Senate seat.

Armstrong became president and CEO of Williams Companies in 2011 and transitioned to executive chairman in 2025. The Tulsa-based company specializes in the collection, storage, and transportation of natural gas and employs about 5,800 people.

Armstrong gave Stitt a total of $8,500 in campaign contributions since 2018. Oklahoma law caps individual campaign contributions to a candidate at $8,500.

Dewey Bartlett Jr., former mayor of Tulsa and president of an oil and gas company, described Armstrong as an even-tempered individual likely to build consensus rather than being driven by political ideology. Bartlett's father served in the U.S. Senate in the 1970s.

"Armstrong's knowledge of the energy industry and its market factors were relevant as disruptions to global oil flows caused by the Iran war continue," Bartlett said.

U.S. Representative Kevin Hern has announced his candidacy for the Senate seat. Stitt's term as governor ends in January.