Johnson, initially the long-shot candidate, beat Ken Livingstone by almost 6%.
The candidate beat expectations in both Indiana and North Carolina, all but locking up the nomination.
Only a few months ago, Democrats were cheering when their candidate beat the Republican in an upstate New York election.
In Arizona, a long-shot candidate beat the speaker of the State House of Representatives for a pivotal seat in the State Senate.
In West Delhi, Parvesh Verma, a debutant 36-year-old BJP candidate, beat a Congress veteran by 460,000 votes out of 1.3m cast.
How does a candidate beat an opponent who, some polls say, holds positions on issues more closely in line with those of most Americans?
In 2006, Romano Prodi, the center-left candidate, beat the center-right Mr. Berlusconi, but with such a narrow majority in the upper house, the Senate, that he had difficulty maintaining his fractious coalition.
Having served in the IPCC since 1992, the only Asian candidate beat five other mostly western nominees to the chairmanship on 6 October, including his closest rival, Belgian professor Jean-Pascal van Ypersele. Jean-Pascal van Ypersele
After almost 70 years as a bastion of the Italian left, its city council has passed into the hands of the anti-establishment Five Star Movement (M5S), whose mayoral candidate beat the centre-left's challenger in a runoff.
The Workers Party candidate beat Aécio Neves, who was widely favoured by the business community, by just 3% of the vote.
