The King and Queen don’t get to America often — their last visit was nearly two decades ago. On Wednesday, the royals honored the victims of 9/11 as the centerpiece of their short stay.

Charles and Camilla performed perfectly, but Mayor Mamdani’s bizarre behavior as host prompts only a question: what the hell is wrong with you?

King Charles and Queen Camilla’s remembrance, meeting with families and laying a wreath at the memorial downtown, is not just a gesture to Americans, months shy of marking the quarter-century anniversary of the terror attacks.

The British and Commonwealth people lost 67 souls on 9/11 — the most of any nation other than our own. The toll was an emblem of our countries’ closeness: executives thought nothing of a quick trip across the Atlantic. The Windows on the World conference atop of One World Trade Center — from which no one escaped — was a part-British event. Some victims marked their only time ever in New York not in days but in hours.

The King’s mother and his heir previously paid their 9/11 respects on New York soil. Queen Elizabeth visited a lower Manhattan garden dedicated to UK victims in 2010. Kate and William visited the 9/11 memorial in 2014. This tradition is becoming an important thread of the US-UK bond.

Mayor Mamdani had just one job: don’t be a jerk.

Instead, the mayor first tried to ignore the King’s visit. Mamdani knew he couldn’t skip the 9/11 appearance — but he omitted the purpose. On his press schedule, Mamdani’s staffers noted only that the mayor would “attend the 9/11 Memorial wreath-laying ceremony” — no mention that the point of the ceremony was so that the King could offer condolences.

Then, hours beforehand, asked by a reporter what he might say to Charles, the mayor purposefully unwelcomed the King. “I’ll be attending a wreath-laying alongside a number of other elected officials,” he said. “The focus of that wreath-laying is to honor the more than 3,000 New Yorkers who were killed.”

Pressed further, Mamdani offered a slap instead of a handshake: “If I was to speak to the king separately from that, I would probably encourage him to return the Koh-i-Noor Diamond,” he said.

No, Mr. Mayor, the King’s wish to honor the dead a few blocks from City Hall was not an opportunity for a freshman barb on colonialism, and imply that this crown jewel belongs to India.

Anyway, what is Mamdani’s standing to demand the diamond — has he appointed himself a representative of India’s government?

It wasn’t just the mayor’s rudeness, but tone and demeanor. Mamdani did his best version of a petulant teen imparting intolerable boredom.

But even the dullest teen would know to say at the press conference: I look forward to joining Charles and Camilla in reflecting on 9/11, including on the British lives lost.

Mamdani did briefly greet Charles at the 9/11 memorial.

It’s a good thing another mayor was on hand, though, to do the apparently arduous task of behaving like a normal person. Michael Bloomberg and the King spent significant time together Wednesday, walking and talking through the memorial’s grounds.

Serving as mayor of a city with so much global history is a big job — it is not all daycare and cheap eggs. On Wednesday, Mamdani made himself look small.

Nicole Gelinas is a contributing editor to the Manhattan Institute’s City Journal.