Mr Khan’s victory broke the Conservatives’ eight-year hold on City Hall.
Labour leader Jeremy Corbyn led congratulations from the party’s MPs, as the bus driver’s son and human rights lawyer won out against Tory rival Zac Goldsmith.
Anne Hidalgo, the mayor of Paris, offered her “felicitations” on Twitter, saying she was looking forward to strengthening co-operation between the two capitals on key issues such as housing, pollution and culture.
Mr Khan, MP for Tooting, was the clear frontrunner from the start of the counting process, which saw details of first preference votes released as they were collated using an electronic system.
And Mr Corbyn did not wait for the official result to claim victory on Twitter, declaring: “Congratulations Sadiq Khan. Can’t wait to work with you to create a London that is fair for all! #YesWeKhan.”
Former Labour leader Ed Miliband offered Mr Khan his congratulations, saying: “You will be an outstanding Mayor of London. Your positive vision and dignity beat a campaign of fear and division.”
Meanwhile, one of the Conservatives’ most senior Muslims conceded defeat with a bitter attack on a Goldsmith campaign which had been widely criticised for using “dog whistle” tactics to brand Khan a “radical”.
Former cabinet minister Baroness Warsi said: “Our appalling dog whistle campaign for London mayor 2016 lost us the election, our reputation and credibility on issues of race and religion.”
Richmond Park MP Mr Goldsmith and Prime Minister David Cameron sought to link Mr Khan with Muslim “extremists” in the bitter race to succeed Boris Johnson.
Mr Cameron sparked Labour fury by twice using Prime Minister’s Questions in the House of Commons as a launchpad for attacks on Mr Khan’s appearance on platforms alongside individuals he branded extreme.
But the tactic backfired after imam Sulaiman Ghani, who Mr Cameron had claimed was a supporter of terrorist group Islamic State, said that he was backing the Tories.
Speaking in Sheffield, Mr Corbyn: “This vile campaign run by the Tories, the way they’ve tried to smear Sadiq Khan, the methods they’ve used and the language they’ve used, has had a very big effect in exactly the way they didn’t want.”