Sen. Alan Armstrong, R-Okla., left, joined by his wife Shelly Armstrong, is sworn in by President Pro Tempore of the Senate, Sen. Chuck Grassley, R-Iowa, right, during a reenactment, to temporarily fill the Senate seat vacated by Markwayne Mullin at the Capitol in Washington, Tuesday, March 24, 2026. (AP Photo/J. Scott Applewhite)

On Tuesday, Oklahoma Gov. Kevin Stitt (R) announced that he will appoint energy executive Alan Armstrong to the U.S. Senate to fill the remainder of Sen. Markwayne Mullins’ (R-OK) term.

The appointment comes after President Donald Trump announced that Mullin would replace Kristi Noem as Department of Homeland Security (DHS) secretary. The U.S. Senate confirmed Mullins’ appointment on Monday and Trump swore him in Tuesday afternoon.

Under Oklahoma law, U.S. Senate appointees in the state are not eligible to seek a full term and are required to sign an affidavit saying they will not do so. Armstrong will serve for ten months until next January, when Mullins’ current term expires. Earlier this month, Trump endorsed U.S. Rep. Kevin Hern (R-OK) in this year’s Oklahoma Senate race and he is likely to secure a six-year term this November, given race ratings.

Armstrong has spent nearly 40 years working at the Tulsa-based Williams Company, an energy firm focused on natural gas processing and petroleum-based energy endeavors. Armstrong served as the company’s president and CEO from 2011-25, during which the company incurred millions in federal penalties for environmental safety violations.

Armstrong’s appointment became official during a press conference on Tuesday, where Stitt said, “Alan’s story reflects the very best of Oklahoma.”

Stitt and Armstrong reportedly met with Trump at Mar-a-Lago over the weekend to discuss the Senate appointment, and possibly Armstrong’s previous campaign donation to former U.S. Rep. Adam Kinzinger (R-IL).

Kinzinger left Congress in 2022 after becoming a vocal opponent of Trump. He was one of the few Republicans in Congress to push back on Trump’s unsubstantiated claims of widespread voter fraud during the 2020 presidential election. He also was one of 10 congressional Republicans who voted to impeach Trump for incitement of insurrection for his actions leading up to the attack on the U.S. Capitol on Jan. 6, 2021.