Trump fires DHS secretary Kristi Noem, replaces her with Sen. Markwayne Mullin

President Donald Trump on Thursday said Republican Sen. Markwayne Mullin will replace Kristi Noem as Department of Homeland Security secretary.

Trump made the announcement on social media Thursday afternoon saying he was "pleased to announce that the Highly Respected United States Senator from the Great State of Oklahoma, Markwayne Mullin, will become the United States Secretary of Homeland Security (DHS), effective March 31, 2026."

"The current Secretary, Kristi Noem, who has served us well, and has had numerous and spectacular results (especially on the Border!), will be moving to be Special Envoy for The Shield of the Americas, our new Security Initiative in the Western Hemisphere we are announcing on Saturday in Doral, Florida. I thank Kristi for her service at 'Homeland,'" Trump said in the post.

This marks the first Cabinet secretary Trump has fired in his second term.

Trump's decision comes as some parts of DHS -- from the Federal Emergency Management Agency to the Transportation Security Administration to the Coast Guard -- are shut down amid a funding fight over Immigration and Customs Enforcement. Democrats have said they will fund the department only if changes are made to the agency in the wake of the shooting deaths of Renee Good and Alex Pretti.

Shortly after the announcement, Mullin told reporters that he was "excited" about leading DHS and "ready to get started." Mullin still needs to be confirmed by the Senate.

"My focus is to keep the homeland secure," he said.

Asked about replacing Noem, Mullin said she "is a friend" who was "tasked with a very difficult job."

Asked by ABC News Senior Political Correspondent Rachel Scott what, if anything, Mullin could learn from Noem's time leading the agency, he said that "there's an opportunity to build off successes, and there's also opportunities to build off things that maybe didn't go quite as planned."

In a social media post Thursday afternoon, Noem thanked Trump for the new appointment.

"In this new role, I will be able to build on the partnerships and national security expertise, I forged over the last 13 months as Secretary of Homeland Security," Noem said.

Several Democrats celebrated the news that Trump had fired Noem.

"Kristi Noem is gone. Good riddance," Minority Leader Hakeem Jeffries said at a news conference, adding that "a change in personnel is not sufficient" to reopen DHS.

"We need a change in policy that has to be bold, dramatic, transformational and meaningful,” he said.

Sen. Chuck Schumer, the body's top Democrat, said in a social media post that "the problems at DHS go much deeper than any one person. They've got to rein in ICE and end the violence."

Minnesota Democratic Gov. Tim Walz, who clashed with Noem over the immigration crackdown in the state, said "Kristi Noem has done a stunning amount of damage and it’s good she’s gone." He called for a "complete overhaul of DHS."

The news that the Oklahoma senator would take over Noem's job came after several sources told ABC News that Trump had called Republicans and top allies asking if he should fire her.

The president had privately expressed deep frustration over Noem's testimony during a Senate Judiciary Committee hearing on Tuesday, sources with direct knowledge of the conversations said.

The sources said the president was upset by a particular exchange when Louisiana Republican Sen. John Kennedy questioned Noem about a DHS taxpayer-funded $220 million ad campaign. Noem repeatedly suggested the president was aware of the campaign and had signed off on it.

A senior administration official tells ABC News that the president did not sign off.

"Absolutely not," the senior administration official said.

Kennedy told reporters Thursday that Trump even called him about it.

"I'm not going to speak for him. You folks know him. You can ask him yourselves, but his, I want to put it this way, his recollection and her recollection are different," Kennedy said.

Tuesday's hearing was just the latest in several incidents that have sparked concern among Trump administration officials and some Republicans on Capitol Hill, sources tell ABC News.

Just days ago, Noem and her top adviser Corey Lewandowski made the decision to temporarily suspend TSA Precheck amid the partial government shutdown -- which later had to be reversed after the White House stepped in, according to sources.

Noem has faced criticism in recent weeks over her handling of ICE operations in Minneapolis after the killings of Good and Pretti by federal law enforcement. She was removed from leading operations in the state following the scrutiny, and Border Czar Tom Homan was sent in to take over.

During hearings on Capitol Hill this week, Noem also refused to apologize for labeling the conduct of Good and Pretti as "domestic terrorism" without evidence.

ABC News' John Parkinson, Lauren Peller, Diana Paulsen and Sarah Beth Hensley contributed to this report.