As the clock struck midnight in New York City, it ushered in the start of a new year — and a new mayor.

Zohran Mamdani, the 34-year-old socialist who delivered a thumping victory in November's mayoral election, was officially sworn in during a midnight ceremony held in a subway station.

"Our campaign was built around listening to the people of New York, and we will govern in the same way," Mr Mamdani wrote on X hours before the ceremony.

"Tomorrow, we get to work."

Zohran Mamdani will have a more public swearing-in on the steps of City Hall. (AP: Yuki Iwamura)

The private, underground oath was the first of two ceremonies planned to mark Mr Mamdani's swearing in, with a public block party outside City Hall expected to attract thousands.

Subway swearing-in

In New York, mayoral terms begin on January 1 after elections.

In a bid to avoid any ambiguity about who oversees America's most populous city, it has become something of a tradition to hold a small midnight swearing-in.

Mr Mamdani chose the Old City Hall subway stop as the site of his midnight oath.

Zohran Mamdani was joined by New York Attorney General Letitia James (left) and his wife Rama Duwaji (right). (AP: Yuki Iwamura)

The station was decommissioned in 1945 and is accessible only a few times a year through guided tours.

Although no longer in use, the station boasts stunning architecture featuring tiled arches.

According to Mr Mamdani's office, the station reflects his "commitment to the working people who keep our city running every day".

Letitia James, a political ally and notable foe of the US president, administered the oath of office. (AP: Yuki Iwamura)

Mamdani said the station represented an era when New York invested in infrastructure meant to improve people's lives, an ambition he said his administration aimed to carry forward.