David Lammy, the son of Guyanese immigrants who grew up poor in working-class London, on Friday became Britain’s chief diplomat, taking the lead on British foreign policy at a time of significant challenges.

Mr. Lammy, 51, has deep ties to the United States, having spent summers with relatives in Brooklyn and Queens and earning a master’s degree at Harvard Law School.

He met Barack Obama 20 years ago at a gathering of Black Harvard alumni, and this year he had dinner with the former U.S. president when Mr. Obama visited London. Mr. Lammy canvassed in Chicago for Mr. Obama during his first presidential campaign, and he has developed a deep network of contacts within the Democratic Party.

In an essay in Foreign Affairs magazine published in April, Mr. Lammy wrote that he would focus on rebuilding ties with the European Union, which have been strained by Britain’s decision to leave the bloc, and that his priority was backing Ukraine.

“Above all else, the United Kingdom must continue supporting Ukraine,” he wrote. “The future of European security depends on the outcome of the war there, and the British government must leave the Kremlin with no doubt that it will support Kyiv for as long as it takes to achieve victory.”

The Labour Party has said that its commitment to NATO is “unshakable” and has vowed to raise its military spending from 2.2 percent to 2.5 percent of gross domestic product “as soon as we can.” On the war in Gaza, the party has said it will push for an immediate cease-fire and for the release of Israeli hostages held there.

In an interview with The New York Times this year, Mr. Lammy said that, if he had the privilege of becoming foreign minister, he would be “very conscious that I’ll be the first — it almost makes me emotional as I say it — the first foreign secretary who is the descendant of enslaved people.”