The oath of office was administered by Eric T. Schneiderman, the attorney general of New York, in a brief ceremony inside the front yard of the mayor's rowhouse in Park Slope, Brooklyn, where Mr. de Blasio stood with his family behind a chain-link fence and beside a bare-limbed tree.

"I want to say to all of you how grateful we are," Mr. de Blasio, who wore a black topcoat and cobalt blue tie, told a crowd of journalists and well-wishers, including the actor Steve Buscemi and Howard Dean, the former governor of Vermont.

"From the beginning," the mayor continued, "this has been our family together, reaching out to the people of this city to make a change that we all needed." He added: "This is a beginning of a road we will travel together."

The ceremony, which precedes a formal inauguration to take place at noon on the City Hall steps, was the culmination of a campaign in which Mr. de Blasio carefully calibrated his image as a fiery populist, intent on easing the disparities of a gilded city, and a proud husband and father, whose biracial family seemed a paragon of multi-cultural New York.

The mayor chose the symbolic location of his own middle-class home, about four miles outside of Manhattan, to recite his oath; it was the same location where he announced his campaign nearly a year ago.

Mr. de Blasio emerged from his house at 12:01 a.m., followed closely by his wife, Chirlane McCray, who wore a flecked black-and-grey jacket, his 16-year-old son, Dante, in a sweater and zip-up jacket, and his 19-year-old daughter, Chiara, who donned a pointy party hat.

Mr. Schneiderman, in a casual coat, stood nearby. He recited a quote from Paul Wellstone, the former Minnesota senator: "It is the belief that extremes and excesses of inequality must be reduced so that each person is free to fully develop his or her potential. This is why we take precious time out of our lives and give it to politics."

Mr. de Blasio then placed his hand on a Bible and recited the oath of office, before hugging his family members, one at a time.

Asked for a $9 filing fee, required by the city to register his new office, Mr. de Blasio opted to pay in cash, holding out the bills for the crowd to see. There was much applause.

The city clerk, Michael McSweeney, approached with several papers for the newly minted mayor to sign.

"Here's the pen, Mr. Mayor," Mr. McSweeney said.

It was the very first time that Mr. de Blasio had been addressed by his new formal title. The mayor, unable to suppress a grin, let out a happy laugh.

Mr. de Blasio concluded the late-night appearance by wishing the onlookers a happy new year. With that, he threw a kiss to the crowd, and walked back into his house.