As the Democratic mayor of the nation's largest city for the past five years, the 58-year-old who first ran as an unapologetic leftist in 2013 coasted to re-election that November.
Despite the old truism that the job of New York mayor is the second toughest in America after that of president, de Blasio -- sometimes nicknamed "Big Bird" for his lanky, 6-foot, 5-inch (1.97-meter) frame -- is one of the few people openly confident of his chances.
While he has been testing the waters for months, travelling to early voting states like Iowa and South Carolina despite a packed mayoral schedule, his campaign has been met with widespread derision.
Democratic polling has been particularly humbling back home for de Blasio, who is up against 22 other Democratic candidates.
An eye-popping 76 percent of New York city voters say he should not enter the 2020 presidential race, compared with just 18 percent who say he should, according to a Quinnipiac University poll from early April.
The mayor who succeeded billionaire Michael Bloomberg on the promise of reducing the city's glaring inequalities has defended his own progressive record, arguing it could be a national model.
He introduced free universal pre-kindergarten and paid sick leave, and early this year rolled out a plan to guarantee health care for all New Yorkers.
Since Donald Trump came to power, de Blasio has denounced the Republican president's hardening of immigration policy and has championed the fight against climate change.
In late April he endorsed a package of laws aimed at protecting New York's environment.
In a prelude to his candidacy, he went to promote them Monday in the lobby of Trump Tower, the skyscraper owned by the president and deemed to be among the most polluting structures in Manhattan.
The event turned into a circus, complete with pro-Trump hecklers who held signs reading "worst mayor ever."
The 'perpetual underdog'
A former supporter of Nicaragua's Sandinistas, de Blasio was closer to liberal Bernie Sanders than to Hillary Clinton in the 2016 Democratic nominations battle, even though he managed Clinton's victorious Senate campaign in 2000.
"There's plenty of money in the world, plenty of money in this city. It's just in the wrong hands!" he said in his State of the City address in January.
De Blasio is married to Chirlane McCray, an African-American woman who for decades identified as a lesbian.
He remains popular in the black community, but Hispanics are divided and whites mostly view him unfavorably.
US media outlets have been damning of his presidential ambitions. Local papers have mocked his lack of charisma, the daily SUV trips to exercise at a gym in his former stronghold of Brooklyn, and his bickering with New York's Democratic Governor Andrew Cuomo.
The longer de Blasio delayed his announcement, the more the taunts increased, and several current and former aides spoke in unusually harsh terms to news outlets about his White House bid.
But the mayor appears to have disregarded their opinions, after confiding to MSNBC recently that the only advice that matters is his wife's.
He also portrays himself as a perpetual underdog, recalling that no one believed in his chances as a former city councilman in 2013, when he won the mayoral race.
"I have spent a lot of time in dead last in many a poll in many a race," he told the New York Times in January.
"It's not where you start" the race that counts, he said. "It's where you end."