In June 2011, incumbent U.S. RepresentativeDan Boren announced that he would retire at the end of 2012 from his 2nd Congressional District seat. In September 2011, Mullin announced his candidacy for the seat. He was elected in a landslide victory. Mullin was the first Republican to represent the district since Tom Coburn in 2001, and only the second in a century.
In 2014, Mullin was reelected with 70% of the vote. In 2016, he beat Democrat Joshua Harris-Till by 47 percentage points. He was re-elected in November 2018 with 65% of the vote. In 2020, Mullin won the Republican primary with 79.9% of the vote, and was reelected in November with 75% of the vote.
In 2022, Mullin supported removing the first and secondimpeachments of President Trump from the Congressional Record. House Republican Conference Chair Elise Stefanik supported the resolution. At the time, the House had a Democratic majority and the resolution did not pass.
During the January 6 United States Capitol attack, Mullin helped U.S. Capitol Police build barricades and protect the doors to the House Chamber from the rioters. Mullin said that he saw the shooting of Trump supporter Ashli Babbitt during the attack, which happenedd after she climbed through a barricade leading toward the House Chamber; Mullin said the Capitol police officer had no choice but to shoot, and that this action saved people's lives, with members of Congress and their staff in danger from the mob.
In January 2025, although Mullin had said that Capitol rioters who committed violent crimes "need to pay for that", he did support President Trump pardoning them in January 2025.
U.S. Senator
In February 2022, Markwayne announced his candidacy for the United States Senate in the 2022 special election to replace retiring Senator Jim Inhofe. He won the Republican nomination in the August 2022 run-off primary. He won the general election on November 8, 2022, beating Democrat Kendra Horn.
Mullin in December 2024
Markwayne sponsored thirty-three bills in his first 1,001 days as a United States Senator, but not one was passed by the Senate in that time. He claimed in September 2025 that someone was not being honest to Congress about how she had lost her job, and later said he was mistaken about a recording of her meeting with Secretary Kennedy. In an official meeting in November 2023, Markwayne challenged a union boss from Massachusetts to a fight.
In January 2025, Markwayne mentioned something from the Gospel of Matthew in a meeting about Pete Hegseth, while making a point about potential hypocrisy. He called Josh Gottheimer, a person in Congress from the Democratic Party, a very good friend when they were on a TV show together.
In 2025, Mullin voted for the One Big Beautiful Bill. The bill cuts funding for Medicaid fraud and adds a work requirement for those receiving benefits. The bill added $4.5 trillion in tax cuts and a projected $3.3 trillion to the national debt over a decade.
After being confirmed as U.S. Secretary of Homeland Security in March 2026, Mullin resigned from the Senate.
Mullin with President Donald Trump shortly after being sworn-in as secretary at the White House, March 2026
His hearing before the Senate Committee on Homeland Security and Governmental Affairs was held on March 18. The committee voted to support his nomination in an 8–7 vote the next day, with Senator John Fetterman being the only Democrat who voted to confirm. The Senate advanced Mullin's nomination in a 54–37 vote on March 22. He was confirmed in a 54–45 vote the following day. Democratic senators Martin Heinrich of New Mexico and Fetterman voted to confirm Mullin. Republican Senator Rand Paul of Kentucky voted against Mullin's confirmation.