On ballot papers across the country have also been candidates for the Scottish and Welsh devolved parliaments, English councils and mayors in other regions.
Behind the volume of elections this year is the coronavirus pandemic, which pushed back several races, including the London mayoral contest, from 2020 to 2021.
While a win for Mr Khan is expected in London, the final result may not be clear until the afternoon of Saturday, May 8 at the earliest due to the voting system used in the capital.
Londoners have indicated both a first and second preference for mayor on their ballots, with counting set to begin on 7am on Friday.
If no candidate at least 50 per cent of first preference votes, all but the top two candidates are eliminated, and counting of second preference votes begins.
Front-and-centre on the minds of Londoners when they voted will have been the impact of the coronavirus pandemic on their city, a key theme of the mayoral campaign.
Mr Khan’s re-election bid has taken employment as its focus, with emphasis on promoting tourism and investment to help more than 300,000 Londoners who lost their jobs during the pandemic re-enter the workforce.
Yet challenger Mr Bailey has been keen to point to the incumbent’s record on crime, with pledges to increase police patrols and boost stop-and-search, particularly with an eye to cutting knife crime in the capital.
This year’s London mayoral race has also seen a large number of outside candidates campaign, including Laurence Fox, the prominent anti-lockdown campaigner and Inspector Lewis actor.
In his bid for mayor, Mr Fox has pledged to “unlock” London from continuing lockdown, stating in an open letter to Londoners his intention to “reclaim your freedom to move” and “freedom to work”.
Aside from their next mayor, residents of the capital cast their vote in London Assembly elections.
The 25-member body is part of the Greater London Authority and scrutinises the activities of the mayor.