Voters were drawn to his stance as the cleanest break from the 12 years of Michael Bloomberg, the outgoing mayor whose policies helped make New York one of the nation's safest and most prosperous big cities but also one that has become increasingly stratified between the very rich and the working class.
"The stakes are so high for every New Yorker. And making sure no son or daughter of New York falls behind defines the very promise of our city," de Blasio said at his victory speech Tuesday, which was held at the YMCA in his Park Slope, Brooklyn neighborhood, a far cry from the glitzy Manhattan hotel ballrooms that usually host election night parties.
"To maintain that greatness and to ensure that our brightest days are ahead of us, we must commit ourselves to progressive ideas that will lift us all," he said.
De Blasio, the public advocate, was trouncing Republican rival Joe Lhota 73 to 24 percent in incomplete, unofficial returns that were on pace to post one of the largest routs in the history of the nation's largest city.
De Blasio will need that political capital to tackle his signature campaign promise: to raise taxes on the wealthiest New Yorkers in order to fund universal pre-kindergarten.
That progressive proposal needs approval from Albany, and neither Gov. Andrew Cuomo, who endorsed de Blasio, nor many state legislatures seem eager to raise taxes as many of them head into a 2014 election year. Throughout the campaign, de Blasio insisted he didn't have a "Plan B" if Albany balked, saying his mandate would persuade lawmakers.
De Blasio reached out to New Yorkers he contended were left behind by the often Manhattan-centric Bloomberg administration and pledged to improve economic, educational and quality-of-life opportunities in minority and working-class neighborhoods.
He will soon make two key administration posts to further that agenda: a new schools chancellor, whom he has vowed will be an educator who will listen more to the concerns of parents, and perhaps most pressingly, a new police commissioner. He has not revealed his choice for the top NYPD job but has said he would not retain current Police Commissioner Raymond Kelly.
He pledged to improve community-police relations by reforming stop-and-frisk, a tactic that allows police to question people deemed suspicious. Its critics, like de Blasio, believe it unfairly targets minorities while its supporters give it credit for helping drive down crime. A federal judge deemed its implementation unconstitutional, though her ruling was thrown out by an appeals court last week.
De Blasio comes to office with the backing of most major unions, but they will soon sit at the other end of the negotiating table as the new mayor will be forced to face a major fiscal crisis.
All of the city's municipal unions have expired contracts and many of their leaders are demanding back pay, which could total $7.8 billion, a payout many economists believe would cripple the city's finances. De Blasio has vowed not to negotiate in public but has said retroactive raises could be difficult to produce.
"Progressive changes won't happen overnight, but they will happen," he said. "There will be many obstacles that stand in our way, but we will overcome them."
De Blasio, 52, will take office on Jan. 1 as the 109th mayor of New York.
If de Blasio's margin of victory holds, it will surpass Abe Beame's 40-point win in 1973 as the largest by a non-incumbent since five-borough elections began in 1897.
President Barack Obama called de Blasio to congratulate him. Lhota called de Blasio to concede about half an hour after polls closed at 9 p.m. Tuesday.
"It was a good fight and it was a fight worth having," Lhota told a crowd of supporters in a Manhattan hotel.
Though polling shows New Yorkers largely approve of Bloomberg's policies, those same surveys revealed the city was hungry for a change.
While registered Democrats outnumber Republicans in the city 6 to 1, the last time a Democrat was elected mayor was 1989, when David Dinkins, de Blasio's former boss, was victorious.
Associated Press writers Jennifer Peltz and Michael Casey contributed to this report. Contact Jonathan Lemire on Twitter JonLemire