De Blasio -- who was sworn in as the city's 109th mayor just after midnight in front of his Brooklyn rowhouse -- was inaugurated more formally at a City Hall ceremony at which an audience estimated at 5,000 listened to guest speakers, including former U.S. President Bill Clinton, and singer and civil rights activist Harry Belafonte.
At the public ceremony, he pledged -- as he had during the 2013 campaign -- to address income inequality and "leave no New Yorker behind."
"When I said we would take dead aim at the tale of two cities, I meant it," he said. "I will honor the faith and trust you have placed in me. And we will give life to the hope of so many in our city. We will succeed ... as one city.
"Our march toward a fairer, more just, more progressive place, our march to keep the promise of New York alive for the next generation -- it begins today," de Blasio said.
The new mayor said he will "ask the very wealthy to pay a little more in taxes" -- the cost of a daily small soy latte -- to fund full-day pre-K programs, the New York Daily News said.
"We do not ask more of the wealthy to punish success," he said. "We do it to create more success stories."
State Attorney General Eric Schneiderman first administered the oath, shortly after midnight, to New York's first Democratic mayor in 20 years, the Daily News reported.
"To everyone, this is the beginning of a road we will travel together," de Blasio said immediately after taking the oath during a sidewalk ceremony.
As his first mayoral act, de Blasio signed an executive order that keeps in place all other existing city orders, a guarantee of continuity during the handing over of power from outgoing Mayor Michael Bloomberg.
De Blasio, campaign manager for former U.S. first lady Hillary Clinton's successful bid for the U.S. Senate in 2000, won a City Council seat in 2001. In 2009, he won his first citywide race to become public advocate.