During that election, he was among the hosts of the How To Win An Election podcast for the Times, while also serving as a trustee of the Design Museum and chancellor of Manchester Metropolitan University.
His name was linked to the chancellorship of the University of Oxford, where he once studied, but it ultimately went to former Conservative leader Lord William Hague.
Sir David Manning, who served as the UK’s ambassador to the US between 2003 and 2007, told BBC Radio 4’s The World Tonight programme that Lord Mandelson was a “very articulate, highly intelligent, extremely experienced operator”.
However, he cautioned that the role would see him coming up against “all sorts of issues that will be contentious and difficult” including climate change, dealing with China and the situation in the Middle East.
Lord Mandelson, 71, told the BBC’s Sunday with Laura Kuenssberg programme in November that nobody had actually spoken to him about the ambassador post.
But he said he was “more in favour of a new relationship rather than a special one” with the US, and “would be very interested indeed in giving advice about trade to whoever is appointed”.
His appointment comes ahead of the inauguration of President-elect Donald Trump on 20 January.
Trump has pledged to impose wide-ranging tariffs on his first day in office, which experts say could cost the UK £22bn.
Sir Keir said on Thursday the UK would “have to make sure that we avoid tariffs” when asked about Trump’s comments, and reiterated that he wanted to improve trade with Washington.
A government source said: “The fact the prime minister has chosen to make a political appointment and sent Lord Mandelson to Washington shows just how importantly we see our relationship with the Trump administration.
“We’re sending someone close to the prime minister with unrivalled political and policy experience, particularly on the crucial issue of trade. He’s the ideal candidate to represent the UK’s economic and security interests in the USA.”
Both the White House and Trump campaign were notified of the appointment in advance.
Speaking to the News Agents podcast in November, Lord Mandelson said the new Trump presidency was going to have a profound impact on the security and economic stability of the rest of world.
“It’s absolutely essential that we establish a relationship with President Trump that enables us not only to understand and interpret what he’s doing but to influence it”, he said.
He added that the Labour government should try to “reconnect” with Trump’s ally and tech billionaire Elon Musk, who has been critical of Sir Keir’s government and has been appointed head of new advisory team the Department of Government Efficiency (Doge).
Lord Mandelson was the Labour MP for Hartlepool from 1992 to 2004, during which time he served as Northern Ireland secretary and business secretary under Blair. He stood down as an MP in 2004 to become a European Commissioner.
He also resigned twice as a minister – once for failing to declare a home loan from a cabinet colleague, and a second time over accusations of using his position to influence a passport application.