He is now Bloomberg's successor, elected by a record margin, supported by a former - and perhaps a future - U.S. president, and hailed as the face of a progressive movement that pledges a significant realignment of the nation's largest city.
De Blasio's improbable climb, which included a stint in an obscure public watchdog post and a stunning political comeback last summer, will reach its pinnacle at noon on a cold New Year's Day when he is sworn in by former President Bill Clinton.
When de Blasio's completes the oath, the second he'll have taken in 12 hours, he will become the first Democratic mayor of New York since 1993 and be poised to enact sweeping changes to a city that became safer and cleaner than ever yet more economically divided during Bloomberg's 12 years in office.
The inauguration was expected to be a joyous day for city Democrats, who outnumber Republicans in the city by a margin of 6-to-1 but have been shut out of power since David Dinkins left office two decades ago.
The party's ascension was underscored by the presence of Clinton and his wife, former Secretary of State Hillary Rodham Clinton, who would be a White House favorite if she runs in 2016.
Both Clintons have ties to de Blasio: The new mayor worked for the former president's administration in the Department of Housing and Urban Development - under now-New York Gov. Andrew Cuomo - and he helped manage Hillary Clinton's successful 2000 U.S. Senate campaign. De Blasio and his wife, Chirlane McCray, are also frequently compared to the Clintons since McCray has long been considered the new mayor's most powerful, if informal, adviser.
McCray stood alongside her husband when he emerged from their modest Park Slope home at 12:01 a.m. Joined by their two children, they stood on a makeshift podium as de Blasio took the oath of office from state Attorney General Eric Schneiderman.
Several dozen neighbors and supporters - as well as actor Steve Buscemi, former Vermont Gov. Howard Dean and Patrick Gaspard, the U.S. ambassador to South Africa - stood in the cold to cheer de Blasio, who then signed the oath and gave the required $9 fee to the city clerk.
The new mayor, the 109th in the city's history, briefly thanked the crowd and pledged that his work would begin immediately.
"I want to thank you for having brought us to this moment," he said standing in the same spot where he launched his then-longshot mayoral bid on an equally frigid day in January. "Many great things are ahead for all of us."
De Blasio, an unabashed progressive who touts his Brooklyn roots, takes office at a crucial juncture for the city of 8.4 million people.
Even as the city sets record lows for crime and highs for tourism, and as the nearly completed 1 World Trade Center rises above the Manhattan skyline, symbolizing the city's comeback from the Sept. 11, 2001, terrorist attacks, many New Yorkers have felt they were not part of the city's renaissance.
De Blasio, 52, reached out to those he contended were left behind by the often Manhattan-centric Bloomberg administration, and he called for a tax increase on the wealthy to pay for universal pre-kindergarten.
He also pledged to improve economic opportunities in minority and working-class neighborhoods and decried alleged abuses under the police department's stop-and-frisk policy. He and his new police commissioner, William Bratton, have pledged to moderate the use of the tactic, which supporters say drives down crime but critics claim unfairly singles out blacks and Hispanics.
De Blasio formerly served as public advocate, the city's official watchdog, and used the obscure and under-funded post to launch his mayoral bid. He was mired in fourth place for much of the primary before the candidacies of several better-known opponents - including Christine Quinn and Anthony Weiner - imploded. He then coasted to a general election rout over his Republican opponent Joe Lhota, a former deputy mayor under Rudy Giuliani.