On Saturday morning in Brooklyn, it was business as usual at FancyFree, a sports bar in Fort Greene. Flags for the soccer team Arsenal flapped in the wind as cheers of “Arsenal! Arsenal!” rang through the neighborhood.

Just before the noon kickoff, fans began screaming. Not for any action on the many television screens, but for Mayor Zohran Mamdani, who strode toward a side entrance to the bar.

Inside, he was greeted by roars as he wove his way through the crowd in a blue-and-red Arsenal jersey worn over his white dress shirt and blue tie. The director Spike Lee welcomed him to their seats with an enthusiastic hug.

Fans, including the mayor, regularly flock from across the city, and beyond, to this bar to cheer on Arsenal. But on Saturday, the stakes were higher than usual: It was the Champions League final. After the Gunners claimed the Premier League title the week before, a Champions League win — something the team had never done before — would pile even more icing on top of the cake.

It was a game the mayor — whose precious attention is usually divided between matters like policy proposals and budgetary concerns — couldn’t miss.

“Arsenal! Arsenal!” the bar’s patrons bellowed. Mr. Mamdani clapped along as the Killers song “Human” began playing. He was surrounded by a security detail and sandwiched between friends, both old ones from before his time as mayor, and newer ones, such as Mr. Lee.