Who is Markwayne Mullin, Trump's new pick for Homeland Security secretary?

US President Donald Trump has announced Oklahoma Senator Markwayne Mullin will be the new Homeland Security secretary, replacing the agency's embattled head Kristi Noem.

The first-term senator, a former mixed martial arts fighter, has been a supporter of Trump and his policies since coming to Washington. The Senate would need to confirm Mullin to the post.

Known for his fiery personality, Mullin will be the second Homeland Security chief in Trump's second administration.

The senator has been complimentary of the Department of Homeland Security's (DHS) efforts to detain undocumented immigrants - a fixture of the Trump administration's policy and a focus for Noem before she was ousted.

"Yes, I'm a Republican, yes I'm conservative," Mullin said on the steps of the Capitol after being surrounded by reporters asking about news of his appointment, noting he would work to enforce the policies passed by Congress even for people who do not agree with him.

"Once I make that transition, my focus is to keep the homeland secure, that's going to be my focus, and we're super excited about this opportunity."

Mullin is among the Republicans who have been critical of the ongoing partial US government shutdown, which has allowed funding to lapse from the DHS.

"What is actually happening right now with the Democrat Party by this political theatre, by shutting down DHS for something that you even admitted yourself can't be done," Mullin told CNN last month. "They're not stopping [Immigration and Customs Enforcement (ICE)] from doing their job."

Should he be confirmed, Mullin will head an agency that has faced increased criticism in recent months, particularly in the wake of two US citizens being killed by federal immigration officials in Minnesota.

He has previously supported Homeland Security policy and the administration's immigration crackdown.

"Obstructing federal law enforcement is a felony. Most Americans follow ICE instructions without thinking twice," Mullin posted on social media following the death of Alex Pretti, who was shot dead by immigration agents in Minneapolis.

"These patriots are doing a difficult job under an 8,000% rise in death threats," he added.

One of Mullin's most talked about moments in the Senate, came early into his tenure when he challenged a union leader to a fight in the middle of a hearing.

The incident occured while the Senate Committee on Health, Education, Labor and Pensions was hearing testimony from Sean O'Brien, president of the International Brotherhood of Teamsters, one of the country's biggest unions in November 2023.

When questioning O'Brien, Mullin brought up a social media post in which O'Brien called the senator a "greedy CEO who pretends like he's self made" and said he should "quit the tough guy act in these Senate hearings".

"You want to run your mouth, we can be two consenting adults, we can finish it here," Mullin said before he stood up, and motioned to take his wedding ring off.

Mullin was then scolded by another member of the committee who told Mullin to sit down.

The pair had previously sparred on social media as well.

Mullin told the union leader to "shut your mouth" during a tense hearing exchange in early 2023.

Months later, O'Brien posted a photo of Mullin on social media and wrote: "You know where to find me. Anyplace, Anytime cowboy."

Mullin replied: "MMA fight for charity of our choice. Sept 30 in Tulsa, Oklahoma. I'll give you 3 days to accept."

The social media posts and committee commentary offer a glimpse of Mullin's fiery personality, something often on display in front of the cameras.

Mullin was sworn in as a US senator in 2023.

Over the past two decades, Mullin, father to six children, worked alongside his wife to grow his family plumbing business, according to his website.