“He will ride above everything and hope it’s delivered,” said Andrew Boff, a Conservative member of the London Assembly. “Sometimes that sounds good, but sometimes you’ve got to care a bit more. You’ve got to actually care about some of the things you’re delivering.”

But for Mr. Johnson, the blunders rarely stuck.

“His mistakes or perceived errors were always seen as evidence of his authenticity,” said Tony Travers, a professor of government at the London School of Economics. Many had expected chaos and were pleasantly surprised when the buses kept running and, after a turbulent start at City Hall, Mr. Johnson proved adept at delegating and chose an able chief of staff.

In 2016, no longer mayor, Mr. Johnson was an architect of the Brexit victory and a favorite to become prime minister. But the job went to Mrs. May, and, to much surprise, she appointed him foreign secretary, partly to keep him away from domestic politics.

Given his role in the Brexit campaign, his reception among European counterparts was chilly and his language proved anything but diplomatic. He compared the former French president, François Hollande, to an officer in a World War II prisoner of war camp; suggested that business would invest in Libya once dead bodies had been cleared away; and recited a colonial-era poem at a Burmese temple.

Perhaps his biggest blunder was at a parliamentary committee in 2017, when he incorrectly told lawmakers that an Anglo-Iranian woman being held in Tehran, Nazanin Zaghari-Ratcliffe, had been teaching journalism to students. That statement was used by the Iranian government to justify their claims that she had been spying.

He lasted two years as foreign secretary and then resigned from the cabinet in July 2018, after undermining Mrs. May’s Brexit strategy and claiming that Britain was “truly headed for the status of colony.”

In his pursuit to become prime minister, Mr. Johnson has adapted his old habits — the theatrics, the polysyllabic put-downs, the outlandish plans — for the Brexit era. Just as he used big-ticket ideas as London mayor to put a gloss on difficult circumstances, so he has tried to deflect from some of the complexities of Brexit, too.

Among other suggestions, he has floated the idea of building another bridge, not across the Thames, but from Britain to France.