LONDON — At the point where the River Thames empties into the North Sea, Boris Johnson wanted to build an airport on an artificial island.
It was 2009, and Mr. Johnson was newly in office as London mayor. To study the airport idea, he tried to enlist a leading scientist, Prof. David King, except Professor King had recently warned about rising water levels on the Thames. Hardly ideal conditions for an airport, he told the mayor, balking at the project while instead offering to look into other uses for the waterway.
So the professor was taken aback when Mr. Johnson publicly named him chairman of a team investigating a Thames estuary airport. Professor King confronted the mayor.
“He said, ‘Oh, silly me, silly me,’ or something like that, and fluffed his hair,” Professor King recalled. The mayor went on: “Ah David, it’ll be all right.”
It was not all right. Like several of Mr. Johnson’s marquee projects as mayor, the island airport plan sucked up millions of pounds in planning fees, but never went anywhere against nearly unanimous opposition and practical hurdles, such as how to put an airport on an internationally protected bird habitat.
Expected to become prime minister this week, Mr. Johnson is deploying the bonhomie and bumbling, confected persona that made him London mayor. He is the darling of rank-and-file Tories hellbent on Brexit. He is the choice of President Trump. He is the ringmaster of a campaign circus that last week featured Mr. Johnson punching the air with vacuum-packed smoked herring.
But despite his charm, many of his colleagues, as well as political analysts, question his competence. During his two terms as London mayor, he read the public mood and aimed millions of pounds the same way, whatever dire warnings his briefing papers contained. His most recent high-profile job, foreign secretary, found him ill at ease in a role that required more gravitas than grandiloquence.
The next prime minister will face perhaps Britain’s greatest peacetime crisis, Brexit, one that turns on the sort of labyrinthine details that Mr. Johnson so avoids. His promise to extract Britain from the European Union by the end of October has left many Britons worrying that he will send the country hurtling toward a potentially calamitous no-deal Brexit.
Even allies acknowledge that Mr. Johnson sees himself as someone focused on the big picture, rather than on the details of governance. He is intuitive and improvisational, allies say, often junking a prepared text when making a speech.
“What he does is he picks up the vibration of the moment, of the day, and then he understands the way to go,” said Ray Lewis, an adviser to Mr. Johnson at London’s City Hall. Mr. Johnson takes a broad view, Mr. Lewis said, in a way that reminded him of what it’s like to daydream about rescuing a love interest from an enchanted castle. The technicalities aren’t the point.
“I never thought about where I got the horse from, or the helmet,” Mr. Lewis said. “I just had the big picture. And Boris will have the big picture.”
After a career as a journalist, Mr. Johnson was elected to Parliament in 2001, where he was enmeshed in some controversy, and was fired from the opposition leadership team, after falsely denying reports of an extramarital affair. He was elected mayor of London in 2008, the first of two terms, and promised to cut crime and curtail extravagant spending. An unusual choice for a city that normally tilts left, he found a following as a Conservative with heterodox views on issues like immigration.
The job of mayor, created in 2000, has limited powers. But his record in City Hall offers a hint of his helter-skelter style, and of the charm that distracted many Britons from his profligate mistakes.
“There are no disasters,” he joked after the controversy involving his affair, “only opportunities. And, indeed, opportunities for fresh disasters.”
His tenure as mayor produced some costly failures, such as the “Boris bus.” Ken Livingstone, Mr. Johnson’s predecessor as London mayor, had scrapped a beloved model of a double-decker bus (equipped with an open-air rear doorway) and replaced it with “bendy buses,” which were longer and roomier. Some Londoners regarded them as modern, soulless monstrosities on the city’s narrow streets.
Tapping into that anger, Mr. Johnson vowed to revive the classic model and even brought back onboard ticket sellers, despite warnings from transport officials that the plan was not economically viable.
Within a few years, the onboard ticket sellers were scrapped, the open-air doorways were sealed shut, and the “Boris bus” came to be known mostly for its sweaty, saunalike interior, not to mention the 300 million pounds, or about $375 million, it burned in the public purse.
“He liked to fly by the seat of his pants on things like this, and he was more than happy to bluff or lie,” said Jenny Jones, a London Assembly member for the Green Party at the time.
Mr. Johnson, at that time already being discussed as a future prime minister, seemed enamored of the status and power of City Hall, but “bored with the whole concept of politics and taking responsibility,” Ms. Jones said. “I wouldn’t trust him to feed my cat.”
Nothing burnished Mr. Johnson’s brand like the London Olympics in 2012. He reveled in the chance to sell Londoners on their city, and to sell himself to the world. But afterward, his decision to convert Olympic Stadium to accommodate a Premier League soccer club left an exorbitant legacy — it costs £8 million every summer to shift the seating to track and field from soccer.
He also pursued a plan to build a “garden bridge” over the Thames, which ended up costing £50 million without a brick ever being laid. When Caroline Pidgeon, an assembly member for the Liberal Democrats, interrogated him about the project, Mr. Johnson tried to brush her off. “I find it very depressing,” he said, “that you persist in this Taliban-like hatred of objects of beauty.”
And then there were the three, 25-year-old German water cannons that he purchased in the name of dispelling possible rioters. He volunteered to be blasted by one himself, to show it was safe, and went forward with the purchases despite warnings from experts that they were ill suited for London.
“It was a strange purchase, and of course, history shows it was a substantial waste of public funds,” said Hugh Orde, a former chief constable in Northern Ireland, who had warned the London Assembly about the plans.
Mr. Johnson did oversee some successes, like the expansion of cycling in London, an idea born under his predecessor, Mr. Livingstone, that the new mayor realized with a rental program that became known as “Boris bikes.” But while the bikes were supposed to pay for themselves, taxpayers have been left on the hook for heavy subsidies.
“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.