"In just 84 days' time London will welcome the world," Johnson said in a victory speech at City Hall just before midnight.
"The world will see a city that is going through a neo-Victorian surge of investment in public transport," he added.
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Johnson, or "Boris" as he is known to Londoners, had won 44.0 percent of the vote in the first round to Livingstone's 40.3 percent.
Since no candidate won the 50 percent needed to avoid a run-off, the second preferences of voters who had chosen eliminated candidates were redistributed and Johnson took 51.5 percent to Livingstone's 48.5 percent.
Johnson said he would "continue to fight for a good deal for Londoners".
And he also paid tribute to his defeated rival as "amongst the most creative and amongst the most original" politicians in Britain.
Then, with characteristic humour, the 47-year-old concluded: "May the fourth be with you!" a joking reference to the date.
Johnson's re-election came as a relief to Conservative Prime Minister David Cameron, whose party took a beating in Thursday's mid-term local elections, capping a bad month for the government after Britain slid back into recession.
Labour's defeated candidate Livingstone, in his own speech, wryly noted Labour's "striking gains" nationally.
"I don't know if the prime minister is watching, but I hope the close result didn't give him indigestion over dinner."
The veteran left-winger, who has dominated London politics for thirty years, said he would not be fighting another election.
