US President Donald Trump has replaced his acting chief of staff, Mick Mulvaney, whose departure had long been rumoured.
He said North Carolina lawmaker Mark Meadows would take over. The change had been expected for weeks.
Mr Trump said Mr Mulvaney would become US special envoy to Northern Ireland.
Mr Mulvaney was perceived to have implicated the president in last year's impeachment inquiry in an off-the-cuff remark at the White House podium.
When Mr Mulvaney gave a rare White House press conference last October, he shrugged off criticism over an alleged corrupt deal with Ukraine by saying: "We do that all the time."
Mr Trump was reportedly outraged by the gaffe.
Mr Mulvaney then walked back his comments in a written statement that said: "Let me be clear, there was absolutely no quid pro quo between Ukrainian military aid and any investigation into the 2016 election."
That same month the chief of staff was seen as having have made another slip-up while attempting to defend the president from criticism over a plan, later cancelled, to hold this year's G7 summit at one of his resorts in Florida.
Mr Mulvaney told Fox News that "at the end of the day he still considers himself to be in the hospitality business", prompting the show's host to point out that Mr Trump was president of the United States, not a hotel executive.
The role of presidential chief of staff, part gatekeeper to the Oval Office and part taskmaster for White House employees, traditionally requires ruthless efficiency and organisation, delivered with a delicate touch. James Baker, nicknamed "the velvet hammer", served under both Ronald Reagan and George HW Bush, and was the model for such a role.
Donald Trump is not, however, a traditional president. He prefers to operate on instinct and improvisation - attributes that have thwarted the best designs of his three previous chiefs.
Reince Priebus, a Republican Party functionary, was unable to control the rivalries and feuds that festered within the White House. John Kelly, the former general, attempted to impose military discipline on the administration - and eventually clashed with the free-wheeling president.
Mick Mulvaney's strategy to "let Trump be Trump" appeared to suit the president, but his missteps during the impeachment investigation eventually sealed his fate.
Now it's Mark Meadows's turn. As a congressman, the affable North Carolinian has been an ardent Trump defender in a job that doesn't require the aforementioned administrative skills.
In an election year, however, vocal support and loyalty - and an ability to demand it from subordinates - may be what the president wants most.
Mr Mulvaney last week made headlines again for accusing US media of only being interested in covering coronavirus because "they think this is going to bring down the president".
In February, Mr Trump said reports that Mr Mulvaney would be fired were "false", insisting he had a "great relationship" with him.
Despite his key White House role, which he assumed in January last year, he was never part of the Trump inner circle.
Shortly after he was picked to be chief of staff in late 2018, video emerged of Mr Mulvaney making a disparaging remark about Mr Trump.
He had said in 2016: "Yes, I am supporting Donald Trump, but I'm doing so despite the fact that I think he's a terrible human being." He had also said Mr Trump was "just as bad" as his opponent, Hillary Clinton.
Mr Mulvaney's appointment as US special envoy to Northern Ireland was welcomed by the First and deputy First Ministers of Northern Ireland, Arlene Foster and Michelle O'Neill.
Mrs Foster said the US has been a "loyal friend" to Northern Ireland and the new appointment "will be important in developing that friendship" while Ms O'Neill said she hoped Mr Mulvaney could build on the work of his predecessors in supporting the peace process and the Good Friday Agreement.
Who is Mark Meadows?
Mr Trump's new chief of staff is a close ally of the president with a record of supporting hard-line conservative causes and climate change denial in the US.
In 2012 he publicly embraced the conspiracy theory that President Barack Obama was not born in the US but instead in Kenya.
As head of a group of conservative politicians called the Freedom Caucus, Mr Meadows lobbied the White House to close down the government's climate change office.
But in 2019 he suggested he was "willing to look at" addressing climate change.
Mr Meadows will now retire from the House of Representatives.
In Friday night's tweet Mr Trump said: "I have long known and worked with Mark and the relationship is a very good one."