It was a hectic Monday morning for Peter Mandelson, who was packing up his home before heading to Buckingham Palace for an audience with King Charles III, on the eve of becoming Britain’s ambassador to the United States.

But the stress of leaving London may not compare to what awaits Mr. Mandelson when he lands in Washington on Wednesday. Few British diplomats have taken on a job as fraught with political risk as Mr. Mandelson’s. His first day at the embassy will coincide with President Trump’s 18th day in the Oval Office — and already, some of America’s sturdiest alliances are teetering.

As he packed boxes, Mr. Mandelson was keeping an eye on Mr. Trump’s latest exchanges with Canada and Mexico, after he had imposed — then paused — sweeping tariffs. The European Union looked to be next in his cross hairs. Mr. Trump was gentler about Britain, suggesting to reporters that a deal “could be worked out,” though he claimed its trade balance with the United States was “way out of line.”

“I’m not going to tell the president his business when it comes to trade,” Mr. Mandelson said in an interview, striking a scrupulously diplomatic tone. But he insisted, “We have a balanced trade relationship with the U.S. It’s balanced in goods; it’s balanced in services.”

For Mr. Mandelson, the trick will be to keep Britain out of Mr. Trump’s line of fire — and to do it at a time when Prime Minister Keir Starmer’s left-of-center government is trying to reset its relationship with the E.U., to which Mr. Trump has long been hostile.

Mr. Mandelson has already been denounced by a faction of Trump loyalists, who failed to scuttle his nomination but may have played into his decision to apologize on Fox News last week for deriding Mr. Trump during his first term as a “white nationalist” and a “danger to the world.”

Mr. Mandelson knows the ins and outs of trade flows: He served as the European Union’s trade commissioner from 2004 to 2008, and then headed Britain’s Board of Trade until 2010. He said he hoped his experience would help him make a persuasive case on behalf of Britain, which runs either an $89 billion trade surplus or a $14.5 billion deficit with the United States, depending on whether one cites British or American statistics. (The difference rests in part on how the two sides treat offshore financial centers like Jersey and Guernsey, which are crown dependencies.)

“If I can use that knowledge to advance some mutual understanding, I will do so,” he said. Still, he was quick to add, “I’m not going to be front-of-house in making arguments against the president’s policies. My job is to act behind the scenes to explain our countries’ policies to each other.”

Such a low profile makes sense in Mr. Trump’s Washington, given the president’s history of clashing with big personalities. But it is out of character for Mr. Mandelson. In a four-decade political career, he thrust himself repeatedly into the spotlight. As a young strategist for the Labour Party, he was known for his ruthless tactics, earning the nickname the Prince of Darkness.”

Since then, Mr. Mandelson, 71, has fallen in and out of favor with successive Labour leaders, from Tony Blair, for whom he was once a trusted adviser, to Mr. Starmer, to whom he became close before his election victory last July.

Mr. Starmer’s decision to appoint Mr. Mandelson to a post as sensitive as ambassador to the United States surprised some in London, given the prime minister’s reputation for caution and Mr. Mandelson’s reputation for swagger and sharp elbows.

A few of those elbows were thrown at Mr. Trump. In addition to his comments in a 2019 interview with an Italian journalist, Mr. Mandelson wrote a column in 2018 for a London paper, The Evening Standard, in which he took Mr. Trump to task for imposing tariffs on China. The president, he wrote, was a “bully and a mercantilist.”

Mr. Mandelson said, “I completely stand by the intellectual argument” about the negative effect of tariffs. But he said he regretted his choice of words about Mr. Trump. He chalked it up to the inflamed atmosphere in Britain, which was then negotiating its exit from the European Union.

“In 2019, I was a bit irate,” he said. “But nonetheless, the words I used about the president were childish and wrong.”

Mr. Mandelson is also under pressure because he is replacing Karen Pierce, a popular envoy known for having built bridges to people in Mr. Trump’s orbit. When his appointment was announced in December, Chris LaCivita, a Republican strategist who comanaged Mr. Trump’s campaign, posted, “replace a professional universally respected Ambo with an absolute moron — he should stay home!”

Among his other critics were Stephen K. Bannon, Mr. Trump’s former chief strategist, and Raheem Kassam, the British-born former editor of Breitbart News in London. Mr. Bannon said he had objected to Mr. Mandelson’s business dealings in China, while Mr. Kassam said he was “Tony Blair’s most effective operator,” which he did not mean as a compliment.

“Whether it’s a rejection of his credentials, or bringing him to heel, it’s a victory,” Mr. Kassam said of the campaign against him.

Still, Mr. Mandelson drew no public opposition from other powerful Trump allies, like the technology billionaire Elon Musk, or hawkish Republicans like Secretary of State Marco Rubio. He would hardly be the first person who once denigrated Trump to win forgiveness.

Mr. Mandelson emphasized his ties to Silicon Valley, which he visited as a minister in 1998 to draw up a white paper on Britain’s competitiveness. He said his experience in China, which deepened during years of running a consulting firm, Global Counsel, would help him nurture joint British and American competition with China.

“Our chief goal must be to win the advanced technology race,” he said. “We can only win that race together, not divided.”

Mr. Mandelson’s defenders say his heavyweight credentials and cosmopolitan manner could appeal to Mr. Trump, who may recognize in him a fellow player. While they do not know each other, they have moved in similar social circles. Mr. Mandelson sits in the House of Lords, the kind of sinecure that Mr. Trump might also appreciate.

“London will be hoping that assigning a political figure of Mandelson’s stature and intellect will be seen as a compliment, and his experience of trade policy in Brussels not an impediment but an asset,” said Peter Westmacott, who was ambassador during the Obama administration.

Kim Darroch, who served as ambassador during Mr. Trump’s first term, said, “I suspect he’s taken this job because he’s addicted to the political game, and this is a way back on to the field. He’s relentlessly engaged in it, relentlessly curious about it, and exceptionally good at it.”

Mr. Darroch, however, offers a lesson in the perils of diplomacy during a Trump presidency. He was forced to resign in 2019 after a London paper published confidential cables in which he offered an unfiltered, unflattering take on the president.

Mr. Mandelson will try to avoid those traps, in part by playing to Mr. Trump as a deal maker. He rejected suggestions that Britain needed to choose between the United States and Europe. Britain, he said, was merely trying to “tidy up” a flawed trade deal negotiated by Mr. Starmer’s Conservative predecessors.

“What we want with the E.U. is complementary to the U.S.,” he said.

“We got a bad deal with the E.U.,” he added. “The president understands what a bad deal looks like.”

A New Zealand woman filed a lawsuit on Monday against Neil Gaiman, the best-selling British author, and his estranged wife, the musician and writer Amanda Palmer, accusing Mr. Gaiman of repeated sexual abuse and Ms. Palmer of “procuring and presenting” the plaintiff and intending to leave her “trapped, vulnerable and penniless.”

Scarlett Pavlovich, who is from New Zealand but now lives in Scotland, claims in the suit, filed in federal court in Massachusetts, New York and Wisconsin, that Mr. Gaiman raped and assaulted her repeatedly while she was working as a nanny and babysitting the couple’s son.

As a result, Ms. Pavlovich “suffered severe emotional distress, physical injuries and economic losses,” the lawsuit said. She also claimed she was not paid adequately for her work for the couple.

Many disturbing details of the allegations against Mr. Gaiman came to light in an explosive New York magazine article last month, as well as in a podcast last summer, in which Ms. Pavlovich identified herself as an accuser of Mr. Gaiman’s. Four other women spoke out about Mr. Gaiman’s abuse in a podcast last summer.

Mr. Gaiman denied the allegations in a statement last month, saying: “I have never engaged in non-consensual sexual activity with anyone. Ever.” Representatives for Mr. Gaiman and Ms. Palmer did not respond to requests for comment on Tuesday.

Ms. Pavlovich first met Ms. Palmer in New Zealand in 2020, when Ms. Pavlovich was 22 years old and homeless, often sleeping on the beach, according to the lawsuit. After Ms. Pavlovich and Ms. Palmer became acquaintances, Ms. Pavlovich started running errands for Ms. Palmer and Mr. Gaiman, sometimes babysitting their son.

Ms. Pavlovich was also dealing with mental health issues and had discussed them with Ms. Palmer, according to the suit.

Ms. Palmer “was clearly aware of the danger that Gaiman posed,” the lawsuit said.

In February 2022, the lawsuit said, Ms. Palmer asked Ms. Pavlovich to babysit for the weekend, splitting time between Mr. Gaiman’s and Ms. Palmer’s separate houses.

On Feb. 4, 2022, Ms. Pavlovich arrived at Mr. Gaiman’s house for the afternoon and looked after the child for about an hour before Mr. Gaiman took the boy to a friend’s house, changing the course of the night, according to the lawsuit. After having dinner with Ms. Pavlovich, Mr. Gaiman urged her to take a bath while he made a phone call, the suit said.

But Mr. Gaiman, who was 61 at the time, returned to the bath — unannounced and naked — and assaulted and raped Ms. Pavlovich, according to the suit. There had been no work call, the suit said.

Ms. Pavlovich objected and told Mr. Gaiman “no” as he proceeded to assault her, the lawsuit said.

Ms. Pavlovich filed a police report accusing Mr. Gaiman of the assault, according to the lawsuit. She said on a podcast last year that the police told her there was not enough evidence to pursue a case.

After the weekend, Ms. Palmer offered Ms. Pavlovich a job as a live-in nanny, with promises that she would be paid and that Mr. Gaiman would help promote Ms. Pavlovich’s writing career, according to the lawsuit. The couple did not pay her, and throughout her employment, Mr. Gaiman continued to sexually assault her, the lawsuit said.

“She was, in effect, an economic hostage to Palmer and Gaiman,” the suit said. The suit also said that Mr. Gaiman told Ms. Pavlovich to call him “master” and that he frequently referred to her as his “slave.”

The suit also claims that Mr. Gaiman knew Ms. Pavlovich was not consenting to their sexual encounters and that during one assault, Ms. Pavlovich “screamed ‘I can’t’ and ‘no’ at the start of the encounter.”