With a mop of blond hair, a bumbling demeanour and a fondness of drifting into Latin, the erudite 51-year-old has presided over the British capital since 2008.
"His eight years as mayor are characterised by success as a celebrity figurehead and almost no mayoral achievements at all," wrote columnist Matthew Parris, formerly a Conservative lawmaker, in The Times newspaper.
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Even the cycle hire system which bears his name -- "Boris bikes" -- was initiated by his Labour predecessor Ken Livingstone.
Johnson's finest hour probably came when he hosted the London 2012 Olympics and he leaves the British capital with a new cross-city rail line well on track -- both secured on Livingstone's watch.
"I honestly struggle to think what his legacy is," said Steve Hilton, Prime Minister David Cameron's former strategy director, in a BBC television documentary on London's Johnson years.
"None of this is to criticise Johnson. The point I'm making is that with such limited powers, it's hard for any mayor of London to leave a legacy," he added, writing in The Guardian newspaper.
The mayor's chief responsibilities are transport, policing, housing, and promoting economic development.
Most public services in the city are delivered by the London boroughs, the 33 local authorities.
"The mayor of London is not as powerful as the mayor of Paris or the mayor of New York," Tony Travers, politics professor at the London School of Economics, told AFP.
When Johnson beat Livingstone to become London mayor in 2008, eight years after the post was created, expectations for his mayoralty "were relatively low, and his record is much better than anybody would have expected", Travers added.
"The city has continued to grow, its economy is very strong compared to the rest of the UK, public transport systems have improved."
As for London's housing crisis, in a city that grows by 100,000 people a year but only builds around a quarter of that number of new homes, "the mayor would have to share some of the responsibility for that but it's also partly the boroughs' responsibility.
"The number of homes built now is higher than in many years; it's just not high enough," said Travers.
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Regardless of his record of achievement, he has done a tireless job of promoting the capital.
"He has had this beautiful and simple message that London is the greatest city in the world. It's not really political," Johnson biographer Andrew Gimson told AFP.
Johnson riffed on his unifying message in a 2011 book on the history of the city, entitled "Johnson's Life of London", where he told of his pride in a city that, he said, had shaped the world.
Such is his popularity that he is able to appeal beyond core Conservative voters and is typically better known by his first name than his surname.
Londoners salute him as he cycles to work at City Hall, his unruly shock of hair poking out beneath his cycle helmet.
Born in New York, Johnson was educated at the elite Eton College before reading Classics at Oxford University.
A journalist who became a member of parliament, the London mayoralty has made Johnson a household name.
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He enjoys constructing elaborate, witty answers to duck questions over whether he wants to succeed the man he calls "Dave", his old Eton and Oxford contemporary Cameron.
"My chances of being PM are about as good as the chances of finding Elvis on Mars, or my being reincarnated as an olive," Johnson once said.
He is backing the "Leave" campaign in Britain's June 23 in-or-out referendum on its European Union membership, in opposition to Cameron, becoming its top figurehead.
Many commentators believe the decision was another shrewd bit of positioning, leaving him in pole position for the top job if Britain votes to leave and Cameron falls.
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