Mick Mulvaney is out as acting White House chief of staff, and top Trump ally Mark Meadows is in, President Trump announced on Twitter Friday night. Mulvaney will become the U.S. Special Envoy for Northern Ireland.

Mulvaney's departure is not entirely unexpected, and the possibility of his departure had been speculated as the impeachment chapter of Mr. Trump's presidency drew to a close. Mr. Trump announced his decision while in Florida. Meadows has been one of the president's staunchest allies in Congress, particularly through the impeachment probe, and had already said this would be his final term in the House.

"I am pleased to announce that Congressman Mark Meadows will become White House Chief of Staff," Mr. Trump tweeted. "I have long known and worked with Mark, and the relationship is a very good one...I want to thank Acting Chief Mick Mulvaney for having served the Administration so well. He will become the United States Special Envoy for Northern Ireland. Thank you!"

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Mulvaney became the White House's acting chief of staff in December 2018, but never shed the "acting" title, despite nearly 15 months on the job. And the White House did not offer any official comment beyond the president's tweet — none of the laudatory statements that in typical times would accompany the exit of the president's righthand man.

Behind closed doors, Mulvaney's relationship with the president had weakened as time went on. But he is perhaps remembered most publicly for a head-turning press conference he gave at the beginning of the Ukraine saga, when he admitted a delay in hundreds of millions of dollars in aid to Ukraine was driven partly by a desire to pressure the country into cooperating with a Justice Department investigation into supposed Ukrainian involvement in the 2016 U.S. election.

The move has been in process for weeks, if not months, according to sources familiar with the situation, and Mulvaney had become less involved in key White House decisions. The timing was spurred by the conclusion of impeachment proceedings and more clarity in the upcoming election, with the White House assuming Joe Biden will be the Democratic nominee.

Mulvaney will treat the envoy post as a part-time position and will be very involved in Trump's re-election campaign, the sources said.

Like Mulvaney, Meadows has been a member of the very conservative House Freedom Caucus, which was founded on Tea Party principles of cutting government waste, spending responsibly and cutting the debt.

Meadows quickly learned how to take advantage of easy access to a president who loves talking on the phone, and swiftly became one of Mr. Trump's closest confidants in Congress. He was one of the president's most vocal messengers during the five-month impeachment saga that consumed Washington, defending the president's Ukraine call and conduct.

Meadows is expected to keep the pro-Trump wings of the House and Senate close to the president in terms of communication and strategy. And Mulvaney and Meadows are friendly with ties to the start of the Freedom Caucus in Congress, so a smooth transition is expected.

"It's an honor to be selected by President Trump to serve alongside him and his team," Meadows said in a statement. "This President and his administration have a long list of incredible victories they've delivered to the country during this first term, with the best yet to come — and I look forward to helping build on that success and staying in the fight for the forgotten men and women of America."

—Major Garrett contributed reporting.