Boris Johnson served as mayor of London from 1 May 2008 until 5 May 2016, being elected in 2008 and reelected in 2012. During his mayoralty, Johnson oversaw the preparations and hosting of the 2012 Summer Olympics and oversaw the cycle hire scheme. He also introduced the New Routemaster buses and the Thames cable car, while promoting the proposed Garden Bridge over the River Thames; the project was later abandoned after Johnson left office. Johnson also banned alcohol consumption on much of London's public transport.

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. During his first term, Johnson was embroiled in several personal scandals, but remained a popular figure in London with a strong celebrity status. In the 2012 mayoral election, he was re-elected mayor, again defeating Livingstone. During his second term he oversaw the 2012 Summer Olympics. In the 2015 general election he was elected MP for Uxbridge and South Ruislip, a position he held from 2015 to 2023, and stepped down as mayor in 2016. He was succeeded as mayor by Sadiq Khan of the Labour Party.

Johnson left office still popular with the people of London, with 52% of Londoners believing he did a "good job" as mayor while only 29% believing he did a "bad job". Following his mayoralty, Johnson became a prominent figure in the successful Vote Leave campaign to withdraw the United Kingdom from the European Union, served as foreign secretary under Theresa May from 2016 to 2018, and served as Prime Minister of the United Kingdom and Leader of the Conservative Party from 2019 to 2022.

In July 2007, Boris Johnson announced his candidacy to be the Conservative candidate for Mayor of London in the 2008 mayoral election and in September was selected after gaining 79% of the vote in a public London-wide primary.

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. Targeting the Conservative-leaning suburbs of outer London, it capitalised on perceptions that the Labour Mayoralty had neglected them in favour of inner London. His campaign emphasised his popularity, even among those who opposed his policies, with opponents complaining that a common attitude among voters was: "I'm voting for Boris because he is a laugh". The campaign of Labour incumbent Ken Livingstone portrayed Johnson as an out-of-touch toff and bigot, citing racist and homophobic language used in his column; Johnson responded that these quotes had been taken out of context and were meant as satire.

In the election, Johnson received 43% and Livingstone 37% of first-preference votes; when second-preference votes were added, Johnson proved victorious with 53% to Livingstone's 47%. Johnson then announced his resignation as MP for Henley.

Johnson was sworn in as mayor on 3 May 2008. Settling into the City Hall mayoral office, Johnson's first official engagement was an appearance at the Sikh celebrations for Vaisakhi in Trafalgar Square. He received criticism during the early weeks of his administration, largely because he was late for two official functions in his first week on the job, and because after three weeks he went on holiday to Turkey. 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.

While Johnson made no major changes to the mayoral system as developed by Livingstone, he reversed several measures implemented by Livingstone's administration.