After a tense day of counting Mr Johnson beat rival Ken Livingstone by 3%, closer than many opinion polls predicted.
After second preference votes were counted Johnson won 1,054,811 votes to Livingstone’s 992,273.
Nationally Johnson’s Conservative party is trailing Labour by as much as 10%.
The result of the election was announced at City Hall by Greater London Returning Officer John Bennett.
Livingstone, who also lost to Johnson in 2008, said the contest would be his “last election”.
Paying tribute to Livingstone, Johnson said was “amongst the most creative and most original” politicians he’s met.
Mr Johnson said he would “I will dedicate myself to making sure that Londoners and above all, young Londoners, are ready to take the jobs that this amazing city creates.”
He also promised to deliver “good deal for Londoners from the government that will help us deliver prosperity for everybody in this city”.
In re-electing Mr Johnson Londoners shrugged off national issues including the UK’s return to recession and the ongoing row over the Government’s links to News International.
Outside the capital it was a different story with the Conservatives losing hundreds of councillors as voters in England, Scotland and Wales turned against them.
Johnson’s victory was widely predicted by opinion polls which in the days before the election suggested he was leading by as much as 12%.
Livingstone led the polls only for a brief period at the start of 2012 when his promised fares cut gained what turned out to be only temporary traction. A series of negative headlines about his challenger’s tax affairs soon saw Johnson regaining the lead.
A YouGov poll for Thursday’s Evening Standard put Johnson 6% points ahead. The final result was far closer with counting lasting until well into the night.
The night also saw Green Party candidate Jenny Jones score a major victory for her party by beating Liberal Democrat rival Brian Paddick into fourth place.
