Noem “has served us well, and has had numerous and spectacular results (especially on the Border!)”, Trump said.
Trump described Mullin as a “MAGA Warrior” and said he will be a “spectacular Secretary of Homeland Security”.
Mullin’s nomination will be subject to confirmation by the Senate, where Republicans hold a majority.
Trump campaigned for the White House on a pledge to remove millions of undocumented migrants from the United States and DHS is the chief enforcer of his deportation policy.
Speaking to reporters, Mullin said his focus will be “to keep the homeland secure”.
“I think there’s opportunities to build off successes,” he said. “And there’s also opportunities to build off things that maybe didn’t go quite as planned.”
Noem, in a post on X, thanked Trump for her new special envoy position and cited what she called “historic accomplishments” at DHS.
“We delivered the MOST secure border in American history,” she said, and “three million illegal aliens have left the US”.
‘Boondoggle’
Democratic lawmakers welcomed Noem’s departure.
“Good riddance,” said Senate Democratic minority leader Chuck Schumer.
Senator Dick Durbin said Noem “was the face of an unpopular, illegal, and outright dangerous mass deportation campaign that claimed the lives of innocent Americans and terrorised many more”.
“Even the President finally conceded she was unfit for the job,” he said.
Senator Adam Schiff of California said Noem “should have been fired a long time ago”.
“It is telling, at the same time distressing, that what seems to have gotten her fired was her PR campaign ... this $220 million boondoggle, this waste of taxpayer resources that largely promoted Kristi Noem.”
Noem came under bipartisan criticism at Tuesday’s Senate hearing.
Senator Thom Tillis, a Republican from North Carolina, called for Noem’s resignation and said her leadership of DHS has been a “disaster”.
Democrats repeatedly asked her to apologise for the deaths of the two Americans killed in Minnesota during protests against the immigration crackdown and for calling them “domestic terrorists”.
Senator John Kennedy, a Republican from Louisiana and staunch Trump loyalist, questioned Noem about the US$220m spent by DHS on television advertising.
Noem said the advertisements had been “effective”.
“They were effective in your name recognition,” Kennedy shot back.
Trump’s former national security adviser, Mike Waltz, was ousted last year over his involvement in a Signal chat in which sensitive information was shared with a journalist.
Noem’s firing comes amid a partial shutdown at DHS.
Democrats oppose any new funding for DHS until major changes are implemented to how the Immigration and Customs Enforcement (ICE) agency conducts its operations.
They have demanded curtailed patrols, a ban on ICE agents wearing face masks and a requirement that they obtain a judicial warrant before entering private property.
- Agence France-Presse