Zohran Mamdani was sworn in as New York City mayor in the early hours of the new year at the historic City Hall subway station, with his wife Rama Duvaji standing by his side, Reuters reports.
The oath, in which he pledged to support the United States Constitution and the laws of New York, was administered by New York State Attorney General Letitia James.
"Happy New Year to New Yorkers, both in this tunnel and above," Mamdani said. "This is truly an honor and a privilege of a lifetime".
Mamdani was a trailblazing candidate, and many in his city of 8 million - some with hope, some with fear - expect him to be a disruptive mayor.
The socialist Democrat's plans for his first day in office today are in line with his policies and priorities, without deviating much from his predecessors, with a formal swearing-in at midnight, followed by a more formal ceremony in the afternoon.
New York law stipulates that the four-year mayoral term begins on January 1 after the election. To avoid any ambiguity about who is at the helm of America's most populous city, it has become tradition to hold a small swearing-in ceremony just after midnight.
Mamdani chose the Old City Hall subway station, which was decommissioned in the mid-20th century and is only accessible a few times a year via guided tours, as the site for the swearing-in.
According to Mamdani's transition team, the subway location reflects his "commitment to the working people who keep our city moving every day."
The 34-year-old former state legislator has promised rent freezes, free busing and child care, building a campaign around affordability issues that some see as the way forward for his Democratic Party nationwide ahead of the midterm elections.
Mamdani inspired a record turnout of more than 2 million voters and received 50% support, with a nearly 10% lead over Andrew Cuomo, who is running as an independent, and significantly ahead of Republican Curtis Sly.