King Charles III worked the House chamber like a stand-up comedian in a joint meeting of Congress in Washington on Tuesday, cracking jokes and winning over his audience in a strained time in American-British relations.

The king’s speech received standing ovations and laughter across both sides of the aisle as he wielded his British humor like the most genial of scepters.

“This is a city which symbolizes a period in our shared history, or what Charles Dickens might have called ‘A Tale of Two Georges,’” he said, pausing as a wave of laughter rolled through the chamber. “The first president, George Washington, and my five-times great-grandfather, King George III,” he continued.

“Please rest assured I am not here as part of some cunning rear-guard action,” he added. Cue more chuckles.

Charles also made a lighthearted reference to the “tradition” in Westminster of taking a member of Parliament “hostage” at Buckingham Palace when a king addresses the assembly, asking Speaker Mike Johnson whether there were “any volunteers for that role here today.” (The custom is rooted in the 1600s, under Charles I, when the relationship between Parliament and the monarchy was particularly fraught. That Charles was beheaded at the end of a civil war.)

There were some gibes about the extraordinary length of British history. Charles noted that the American colonies declared independence “250 years ago, or as we say in the United Kingdom, just the other day,” adding a pause and smiling at the jubilant response.

The king’s easy demeanor and wit relaxed the gathered lawmakers, who are more often that not at odds with one another. Charles was doing a delicate balancing act. He looked not only to calm the troubled political waters between his country and the United States, but also to gently push back on Mr. Trump’s verbal attacks on Britain for not doing more to help the United States in the war in Iran.

The king received a standing ovation when he referenced Magna Carta, the document King John was forced to sign in 1215 that has become a historic symbol of an individual’s rights against despotic rule. He hailed the importance of the foundations of democracy, including checks and balances of power and thoughtful debate in the legislative process.

His comments came as Congress had surrendered much of its authority to Mr. Trump. Republican lawmakers handed over the legislative branch’s spending authority to the president as he imposed tariffs on U.S. allies and slashed federal programs funded by Congress. Lawmakers have also spent much of the session battling through gridlock to pass any legislation.

Moments like this are an opportunity for the monarchy to show the role it can play in British diplomacy, particularly in recent years, thanks in part to Mr. Trump’s fondness for the royal family.

And the jests served to soften up his listeners for what, in another’s delivery, might have seemed like a lecture. Charles said, “I believe with all my heart, that the essence of our two nations is a generosity of spirit, and a duty to foster compassion, to promote peace, to deepen mutual understanding, and to value all people of all faiths and of none.”

Megan Specia contributed reporting.