By STEPHEN CASTLE and SEWELL CHAN From New York Times

LONDON — After a startlingly swift transfer of power that made her Britain’s prime minister, Theresa May took charge of a new government on Wednesday, vowing to honor the referendum to leave the European Union and naming Boris Johnson, the former London mayor who had been widely considered politically dead, as foreign minister.

Mr. Johnson had been at the forefront of the so-called Leave campaign, but after its triumph abruptly pulled out of the contest to succeed Prime Minister David Cameron, who left office earlier Wednesday.

Speaking as she arrived at the prime minister’s office at 10 Downing Street, Ms. May, 59, who had served for six years as home secretary, sought to position herself firmly in the tradition of “one nation” Conservatism, stressing her commitment to helping the underprivileged and pledging to fight “burning injustice.” She also promised to preside over an economy that benefits everyone.

“As we leave the European Union we will forge a bold new positive role for ourselves in the world, and we will make Britain a country that works not just for a privileged few but for every one of us,” Ms. May said, as her husband, Philip, stood nearby.

Another notable appointment was that of David Davis, a former minister for Europe and a longstanding Euroskeptic, to lead a new department responsible for exiting the European Union. Liam Fox, a former defense secretary, will take charge of international trade.

Ms. May has effectively handed Mr. Johnson, Mr. Davis and Mr. Fox the responsibility for successfully carrying out the withdrawal — known as Brexit — which they repeatedly promised would yield new opportunities.

Putting crucial international portfolios firmly in the hands of Brexit supporters was perhaps an astute move for Ms. May, who had argued, tepidly, for Britain to remain in the European Union.

Not only do the appointments appear to reflect a desire to unite her party around the outcome of the June 23 vote, they may also be calculated to head off any suggestion from right-wingers that she might not follow the will of the voters.

Even so, Mr. Johnson’s elevation to such a prestigious post represents a remarkable change of fortunes in a career that has had its share of them. He played a key role in the extraordinary national dramas of the last few months, abandoning Mr. Cameron, who desperately tried to keep Britain in the European Union, and cheerfully becoming the public face of the Brexit campaign.

Yet when Mr. Cameron announced his resignation after the referendum, Mr. Johnson’s hopes of succeeding him as Conservative Party leader were undermined by another Brexit supporter, Justice Secretary Michael Gove, who threw aside his support for Mr. Johnson and announced his own candidacy. Mr. Gove argued that Mr. Johnson was not up to the job.

Ms. May herself recently mocked Mr. Johnson’s negotiating skills over London’s purchase of used German water cannons when he was mayor. She blocked the use of the cannons, citing fears that they could cause serious injuries.

Among other appointments, Ms. May promoted Amber Rudd, former energy secretary, to home secretary. That ensures that one of the biggest political offices — and Ms. May’s old job — remains in the hands of a woman. The new prime minister chose Philip Hammond, who had been foreign secretary, as chancellor of the Exchequer, replacing George Osborne, a close Cameron ally.

Ms. May moved into 10 Downing Street after a day of political ritual that saw Mr. Cameron address lawmakers for the last time as prime minister, before tendering his resignation to Queen Elizabeth II.

Only on Monday did Ms. May learn that she would become prime minister, when the last remaining contender to lead the governing Conservative Party, Andrea Leadsom, quit the race. Ms. May takes over at a time of acute political turmoil and economic uncertainty and is the 13th prime minister to serve this queen — the first was Winston Churchill.

The Brexit referendum divided the nation, with the majority of voters in a number of less affluent areas opting to quit the bloc, while most of those in London, Scotland and Northern Ireland took the opposite view. On Monday, Ms. May outlined some of the economic changes she hopes to make, speaking about taming excessive executive pay, and arguing that big multinational companies must pay their fair share in taxes.

Just an hour or so before Ms. May spoke outside 10 Downing Street on Wednesday, Mr. Cameron stood in the same spot, flanked by his wife, Samantha, and their three children, paying tribute to his family and his key staff members who had supported him.

“It has been the greatest honor of my life to serve our country as prime minister over these last six years, and to serve as the leader of my party for almost 11 years,” Mr. Cameron said. “My only wish is continued success for this great country that I love so very much.”
Mr. Cameron cited the nation’s economic recovery as his top legacy. “With the deficit cut by two-thirds, two and a half million more people in work and one million more businesses, there can be no doubt that our economy is immeasurably stronger,” he said.

He also cited among his accomplishments the legalization of same-sex marriage, in 2013; changes to the education system; and reduced wait times for operations in Britain’s National Health Service.

“I’m delighted that for the second time in British history, the new prime minister will be a woman, and once again a Conservative,” Mr. Cameron said. “I believe Theresa will provide strong and stable leadership in fulfilling the Conservative manifesto on which we were elected, and I wish her well in negotiating the best possible terms for Britain’s exit from the European Union.”

Hours earlier, in his final parliamentary duty, Mr. Cameron took part for the last time in prime minister’s questions, the weekly ritual in which lawmakers interrogate the leader in often combative exchanges.

On Wednesday, the discussion was more respectful — and lighthearted — than usual, as Mr. Cameron’s political adversaries and allies paid tribute to him.

“I’m told that there are lots of leadership roles out there at the moment: there’s the England football team, there’s ‘Top Gear,’ there’s even across the big pond the role that needs filling,” Danny Kinahan, a lawmaker from Northern Ireland, told Mr. Cameron jokingly, referring to Britain’s soccer team, a wildly popular BBC television show — and the United States presidential election.

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