Croydon’s Conservatives defied London’s pattern by becoming the borough’s first mayor.
The traditional cabinet system, in which councillors choose their leader from among their ranks, has been replaced by a directly elected mayor.
Conservative Jason Perry and Labour candidate Val Shawcross were separated by less than 600 votes.
Mr Perry may have to work with a Labour majority if he wins, which might result in London’s first council power split.
The full council election results are due on Sunday.
In his acceptance speech, Mr Perry offered a poignant homage to his mother, who died during the campaign.
After a run-off between Conservative and Labour candidates and a final recount, it had been an unexpectedly long and arduous count, and daylight had broken by the time the final result was announced.
Labour’s Val Shawcross received 6,617-second choices in a close race, giving her a total of 38,023.
In the run-off, Mr Perry received 5,199-second preference votes, but he began with more first preference votes and concluded with a total of 38,612 votes.
As he waited for the final pronouncement, he had to turn away from the audience on stage to compose himself, and he grew tearful during his victory speech, in which he praised his late mother for her support during the campaign.
As he walked off the platform, he hugged his wife Melanie and son William, who shed a tear as he thanked his father and told him, “She would be so pleased.”
After the borough’s new councillors were elected in a postponed count later on Saturday, Croydon’s new mayor said he was “quite emotional, very excited, and very much looking forward to getting to work.”
Mr Perry also backed Boris Johnson, even though his party lost council seats in Westminster, Wandsworth, and Barnet.