Here are some of the most contentious episodes in the life of the bombastic 55-year-old former foreign secretary and London mayor, elected Britain’s new leader on July 23:
Fired for lying
After graduating from Oxford University, Johnson landed a trainee reporter job at The Times newspaper in 1987.
But he was dismissed within a year for concocting a quote in an article about king Edward II and the monarch’s suspected gay lover.
“It was awful — I remember a deep, deep sense of shame and guilt,” he told BBC television when recalling the incident in 2013.
Fired for lying (again)
In 2004 Johnson, by now an MP, was sacked from his roles as shadow arts minister and Conservative Party vice-chairman by then-leader Michael Howard for lying about an extramarital affair.
Johnson — then married to his second wife, with whom he has four children — had initially dismissed the tabloid allegations of a four-year fling with another woman as “an inverted pyramid of piffle”.
Howard sacked him — as a matter of “personal morality”, according to Conservative officials — due to his lack of candidness following further allegations that Johnson had paid for his mistress’s abortion.
“It is a wretched and lamentable day when people’s private lives can become used in political machinations,” Johnson said at the time.
Johnson’s failed bid to build a so-called garden bridge over the River Thames is seen as perhaps the most egregious error of his eight years as London mayor from 2008.
He oversaw attempts — costing a whopping £53.5 million — to create the new central London crossing covered with trees and flowers, but faced spiralling costs and opposition from local residents.
His successor Sadiq Khan scrapped the project in 2017 on the advice of an independent review.
Johnson later claimed “not a single penny of taxpayers’ money” was wasted on the plan while he was in office and blamed Khan for its demise.
The ex-mayor also faced stinging criticism for wasting more than £300,000 on three crowd-control water cannons that the government then barred from use out of fear that they were excessively heavy-handed.
Accused of Brexit ‘lies’
Johnson, one of the most prominent faces in the 2016 Brexit referendum campaign, helped to promote the controversial claim that Britain would no longer have to make weekly payments of £350 million ($436 million, 388 million euros) to the EU.
Use of the figure, which was emblazoned on the side of the Leave campaign’s touring bus, has been criticised as misleading because it represented the country’s gross contribution to the 28-nation bloc.
The net figure is far smaller because it also includes a budget rebate from Brussels as well as payments to Britain’s public sector from EU coffers.
“Get that lie off your bus,” he was told by a rival during a TV debate.
Johnson has nonetheless stood by the claim, telling a 2017 radio phone-in that it “represents the total sum that we do not control every week that is spent by Brussels”.
Johnson’s 2016 to 2018 tenure as foreign secretary featured a number of gaffes — the most high-profile involving a British-Iranian, Nazanin Zaghari-Ratcliffe, who is being held in a Tehran jail.
The dual citizen, who worked for the Thomson Reuters Foundation — the media group’s philanthropic arm — was detained in 2016 as she left Tehran after taking her infant daughter to visit her family.
She was later jailed for five years for alleged sedition. She vehemently denies the charges.
During a 2017 hearing in the British parliament, Johnson stated that Zaghari-Ratcliffe had been training journalists in Iran, in what he later described as a “slip of the tongue”.
Iran’s judiciary promptly seized on the comments as proving that she was not on holiday, and Johnson was forced to call his Iranian counterpart to try to clarify the remarks.
He apologised in Britain’s parliament, retracting “any suggestion she was there in a professional capacity”, but resisted calls to resign over the error.