US President Donald Trump announced Friday he had chosen staunch ally Mark Meadows to be his new chief of staff -- the fourth person to hold the position since he took office.

Meadows, a Republican congressman from North Carolina, will replace the ultra-conservative Mick Mulvaney, who had been serving as acting chief of staff since Trump fired John Kelly in December 2018.

"I have known and worked with Mark, and the relationship is a very good one," Trump tweeted, making the announcement. 

"I want to thank Acting Chief Mick Mulvaney for having served the Administration so well," Trump added. "He will become the United States Special Envoy for Northern Ireland."

In December, Meadows, 61, announced he would retire from Congress. But "my work with President Trump and his administration is only beginning," he said in a statement posted to his website.

In October, in the midst of the impeachment proceedings against Trump, Mulvaney was criticized for his admission during a press conference that the president had tied military aid for Ukraine to Kiev opening a probe into the Democrats.

"We do that all the time with foreign policy," Mulvaney told reporters. "Get over it. There is going to be political influence in foreign policy."

He walked back that admission only hours later.

The chief of staff is a key position in the White House, acting as the president's right-hand person and the coordinator of administrative action.

Trump's first chief of staff was Reince Priebus, the former leader of the Republican party, who left the position before he had even passed the 200-day mark.