The Conservative frontrunner is now facing a possible private prosecution for intentionally misleading voters during the EU referendum campaign.
And while he once told The Independent that his mistakes “are too numerous to list in full”, here is our roundup of his seven most notorius untruths.
The made-up quote
Boris Johnson was sacked from his job at The Times newspaper over allegations he fabricated a quote from his godfather, the historian Colin Lucas, for a front-page article about the discovery of Edward II’s Rose Palace.
“The trouble was that somewhere in my copy I managed to attribute to Colin the view that Edward II and Piers Gaveston would have been cavorting together in the Rose Palace,” he claimed.
Alas, Gaveston was executed 13 years before the palace was built. “It was very nasty,” Mr Johnson added, before attempting to downplay it as nothing more than a schoolboy blunder.
Fake news from Brussels
After leaving the Times, Mr Johnson moved to The Daily Telegraph, working as the publication's Brussels correspondent between 1989 and 1994.
His articles, like those in several other Eurosceptic newspapers, contained many of the claims widely described as “Euromyths”, including plans to introduce same-size “eurocoffins”, establish a “banana police force” to regulate the shape of the curved yellow fruit, and ban prawn cocktail crisps.
He was questioned about this claims while London mayor, but denied suggestions they were a figment of his imagination.
“There is a great deal of effort being made to deprecate those who think we should leave the EU and everything we say is somehow mythical”, he replied.
Misrepresenting the people of Liverpool
Mr Johnson became editor of the Spectator in 1999 after telling owner Conrad Black, who was later convicted of fraud, that he would not pursue a political career. This promise was broken in 2001 when he won election as Conservative MP for Henley in Oxfordshire.
Three years later he was forced to apologise for an article in the magazine which blamed drunken Liverpool fans for the 1989 Hillsborough disaster and suggested that the people of the city were wallowing in their victim status.
“Anyone, journalist or politician, should say sorry to the people of Liverpool – as I do – for misrepresenting what happened at Hillsborough,” he said.
The extramarital affair
Michael Howard gave Boris Johnson two new jobs after becoming leader of the Conservatives in 2003 – party vice-chairman and shadow arts minister.
He was sacked from both positions in November 2004 after assuring Mr Howard that tabloid reports of his affair with Spectator columnist Petronella Wyatt were false and an “inverted pyramid of piffle”. When the story was found to be true, he refused to resign.
The Mayor’s flirtation with fallacy continued as London Mayor. Having promised in his 2008 manifesto to ensure there would be manned ticket offices at every train station, he agreed to widespread closures to pay for a 24-hour tube.
He promised to eradicate rough sleeping by 2012, only for it to double during his leadership. He was also accused of telling “barefaced lies” after he stated that police numbers would increase in London despite government cuts.
The lies on the bus go...
Launching the Vote Leave bus tour, Mr Johnson returned to the scene of his earlier falsehoods by repeating his old allegations that the EU was setting rules on the shape of bananas.
He also backed the infamous claim on the side of the bus that the UK was sending £350m a week to the EU, followed by “let’s fund our NHS instead”.
The UK Statistics Authority issued an official statement in May 2016 describing the claim as “misleading”, but Mr Johnson repeated it in an article in the Telegraph in September 2017.
The article has since been taken down and Mr Johnson is facing a private prosecution over claims he deliberately lied during the campaign.
The MP's lawyer told a court: “I should make it clear that because of the interest in this case that it is absolutely denied by Mr Johnson that he acted in an improper or dishonest manner at any time.”
‘I didn’t say anything about Turkey’
In January Boris Johnson claimed he did not mention Turkey during the referendum after it was suggested he falsely claimed 80 million Turks would come to Britain unless the UK left the EU.
In fact, he co-signed a letter stating that “the only way to avoid having common borders with Turkey is to vote Leave and take back control”. The Vote Leave campaign also produced a poster reading: “Turkey (population 76 million) is joining the EU”, adding “David Cameron wants Turkey to join the EU. How will our NHS cope?”.
Mr Johnson, whose great-grandfather was the Ottoman politican Ali Kemal, was also quoted as saying “I am very pro-Turkish but what I certainly can’t imagine is a situation in which 77 million of my fellow Turks and those of Turkish origin can come here without any checks at all. That is mad – that won’t work.”
Mr Johnson’s Turkish cousin commented: “He doesn’t strike me as being very honest about his views.”