[1] Selected as the Conservative candidate for the London mayoral election of 2008, Johnson defeated Labour incumbent Ken Livingstone and resigned his seat as MP for Henley in parliament.

During his first term as mayor, he banned alcohol consumption on public transport, introduced the New Routemaster buses and cycle hire scheme, and championed London's financial sector.

[5][6] Johnson's mayoral campaign focused on reducing youth crime, making public transport safer, and replacing the articulated buses with an updated version of the AEC Routemaster.

[8] His campaign emphasised his popularity, even among those who opposed his policies,[9] with opponents complaining that a common attitude among voters was: "I'm voting for Boris because he is a laugh".

[15][16] Settling into the City Hall mayoral office,[17] Johnson's first official engagement was an appearance at the Sikh celebrations for Vaisakhi in Trafalgar Square.

[19] In July 2008, Johnson visited the closing ceremony of the 2008 Summer Olympics in Beijing, where he offended his Chinese hosts with his attire by failing to button his suit jacket.

[23] During the first Mayoral term, Johnson was perceived as having moved leftward on certain issues, for instance supporting the London Living Wage and endorsing an amnesty for illegal migrants.

[24] He tried placating critics who had deemed him a bigot by appearing at London's gay pride parade and praising ethnic minority newspapers.

[25] In 2012, he banned London buses from displaying the adverts of Core Issues Trust (a Christian group) which compared homosexuality to an illness.

[39] Johnson resigned as MPA chairman in January 2010,[24] but throughout his mayoralty was highly supportive of the Metropolitan Police, particularly during the controversy surrounding the death of Ian Tomlinson.

[41][42] Similarly, his claim that Metropolitan Police numbers had increased was also characterised as untrue,[41] but the fact-checkers at Full Fact say that both Johnson's and his critics' positions are defensible.

[64] Controversy erupted when he was questioned about his Telegraph fee on BBC's HARDtalk; here, he referred to the £250,000 as "chicken feed", something that was widely condemned, given that this was roughly 10 times the average yearly wage for a British worker.

[71] He was subsequently accused of failing to publish an independent report on air pollution commissioned by the Greater London Authority, which revealed that the city breached legal limits on nitrogen dioxide levels.

[49] He introduced a public bicycle scheme that had been considered by Livingstone's administration; colloquially known as "Boris Bikes", the partly privately financed system cost £140 million and was a significant financial loss but proved popular.

[79][80] At the beginning of his tenure as mayor, Johnson announced plans to extend pay-as-you-go Oyster cards to national rail services in London.

[92] He was accused of cronyism,[93] in particular for appointing Veronica Wadley, a former Evening Standard editor who had supported him, as the chair of London's Arts council when she was widely regarded as not being the best candidate for the position.

[103] Polls suggested that while Livingstone's approach to transport was preferred, voters in London placed greater trust in Johnson over issues of crime and the economy.

[105] The political scientist Andrew Crines believed that Livingstone's campaign focused on criticising Johnson rather than presenting an alternate and progressive vision of London's future.

[112][113] Johnson was accused of covering up pollution ahead of the games by deploying dust suppressants to remove air particulates near monitoring stations.

[116] Johnson had a close friendship with American technology entrepreneur, former DJ[117] and model Jennifer Arcuri, with The Sunday Times describing him as a regular visitor to her flat,[118] and implying they were in a sexual relationship.

[123] The IOPC issued its report in May 2020, concluding that, although there was no basis for any criminal charge, there was evidence that decisions by officials had been influenced by the close relationship between Johnson and Arcuri.

[132] In February 2012, Johnson criticised London's Saint Patrick's Day gala dinner celebrations, linking them to Sinn Féin and branding the event "Lefty crap",[133] for which he later apologised.

Upon realising that the vote meant that he would not be questioned on the budget, Johnson referred to his political opponents as "great supine protoplasmic invertebrate jellies".

[135] Johnson attended the launch of the World Islamic Economic Forum in London in July 2013, where he answered questions alongside Malaysian Prime Minister Najib Razak.

"[139][140] Also in 2014, he was criticised for saying that "almost half" of his senior staff were female, when London Assembly members stated that only four of fourteen top positions in Johnson's administration were occupied by women.

[141] In 2015, Johnson criticised then-presidential candidate Donald Trump's false comments that there were no-go zones in London governed by shariah and inaccessible for non-Muslims.

[2] In 2016, Sadiq Khan announced that three German-made water cannon, which Johnson had bought for the Metropolitan Police without waiting for clearance from the then-Home Secretary Theresa May, were to be sold off with the funds going to youth services.

[150] Following his mayoralty, Johnson became a prominent figure in the successful Vote Leave campaign for Brexit in the 2016 European Union (EU) membership referendum.

Parliament ratified Johnson's Brexit withdrawal agreement, and the UK left the European Union on 31 January 2020, beginning an eleven-month transition period.

In March 2020, Johnson responded to the COVID-19 pandemic by enacting emergency powers and widespread societal measures including several lockdowns, and approved a vaccination programme.