The 52-year-old public advocate and his black formerly lesbian wife promise a new style in a city transformed by 12 years of tough love under Bloomberg, who is stepping down after a record three terms.
De Blasio's campaign has left Republican rival Joe Lhota trailing in the dust, addressing the concerns of the economically vulnerable middle class and tapping into a far larger Democratic electorate.
He went into the election with an historic 41-point lead over Lhota in an NBC-Wall Street Journal poll, putting him on course to be the first Democrat elected mayor of the biggest US city since 1989.
He was expected to speak to supporters later today.
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The city of 8.3 million has six times as many Democrat voters yet David Dinkins was the last mayor to win the race in 1989.
If elected New York's 109th mayor, de Blasio promises to raise taxes to fund universal pre-kindergarten education and after school programs, and build 200,000 affordable housing units.
He wants to reform the "stop and frisk" policy, which critics say unfairly targets black and Hispanic minorities, but which supporters say has driven down crime.
His campaign has raised concerns about the gulf between rich and poor in a city with more than 440,000 millionaires but where 21 per cent live in poverty on USD 30,944 a year for a family of four.
Lhota was upbeat when he cast his own ballot, accompanied by his wife in upmarket Brooklyn Heights.
He is a former deputy mayor under Rudy Giuliani and a respected former chairman of the city's Metropolitan Transportation Authority.