Curtis Sliwa, the Guardian Angels founder, flamboyant radio host and Republican nominee for mayor, has been an inescapable fixture of life in New York City for decades.

But when he strolled into the Lower Manhattan offices of an important business group recently, its chief executive literally did a double take.

Mr. Sliwa had swapped out his familiar sateen Guardian Angels jacket for a dark suit. And on his head, where a swooping red beret has sat almost every day of his adult life, there was only a cap-shaped tan line and balding pate.

“He stuck out his hand, and I looked at him and said, ‘Oh my god!’” said Kathryn S. Wylde, the longtime leader of the group, the Partnership for New York City. “‘I didn’t recognize you.’”

In a city rich with sartorial symbols, few have been more memorable than Mr. Sliwa’s ruby red headpiece. It helped the Guardian Angels, his subway patrol group, gain notoriety in the 1970s; was his uniform for a career in television and radio and provided an unofficial motif for his unsuccessful first run for mayor in 2021.

Yet as he takes a second, seemingly more viable run at City Hall, Mr. Sliwa, 71, is beginning to show up without it. Certainly not always, but especially for meetings with business leaders, union officials and others he has deemed to be serious people. He has pledged to keep it off permanently if he is elected in November.