Khan's victory, his third in a row, was widely expected despite some public anger over knife crime and the Ultra Low Emission Zone (ULEZ), which charges drivers of older, more polluting vehicles a daily fee.
For Labour, London is the latest of a number of councils and mayoralties it has won in the local elections, which took place on Thursday, inflicting heavy losses on Prime Minister Rishi Sunak's Conservatives.
"It's been a difficult few months, we faced a campaign of non-stop negativity," Khan said in a speech after the results showed he had won 43.8 per cent of the vote against 33 per cent for the Conservatives' candidate, Susan Hall.
"For the last eight years, London has been swimming against the tide of a Tory (Conservative) government and now with a Labour Party that's ready to govern again under Keir Starmer, it's time for Rishi Sunak to give the public a choice."
Opinion polls predict that Labour will win the next general election, propelling its leader Keir Starmer to power and ending 14 years of Conservative government in the UK.
Khan, 53, who became the first Muslim mayor of the British capital in 2016, has pledged to build more social housing and work with a future UK Labour government to boost police capacity.
Khan, who replaced Boris Johnson as London mayor in 2016 and who has widespread policing and budget powers, has been an increasingly divisive figure in the past few years, particularly in the suburbs, where he fared worse than in the inner city.
His supporters say he has multiple achievements to his name such as expanding house-building, free school meals for young children and keeping transport costs in check.
His critics say he has overseen a crime surge, been anti-car and has unnecessarily allowed pro-Palestinian marches to become a regular feature at weekends.
The incumbent Labour mayors in Liverpool, Greater Manchester and West Yorkshire were also re-elected on Saturday while the party may have ousted the Conservative mayor in the West Midlands, arguably the bellwether region of the UK, where a recount is underway.
By late afternoon on Saturday, with most of the 2661 seats up for grabs in the local elections counted, the Conservatives had lost about a half of the 1000 seats they were defending while Labour had picked up about 200 despite some seemingly Gaza conflict-related losses.
with AP
