Zohran Mamdani became mayor of New York City just after midnight on Thursday, taking the oath of office at an historic, decommissioned subway station in Manhattan.

The Democrat was sworn in as the first Muslim leader of America’s biggest city, placing his hand on a Quran as he took his oath. “This is truly the honour and the privilege of a lifetime,” he said.

The ceremony, administered by New York attorney general Letitia James, a political ally, took place at the old City Hall station, one of the city’s original subway stops known for its stunning arched ceilings.

Mamdani will be sworn in again, in grander style, in a public ceremony at City Hall at 1pm by senator Bernie Sanders, one of the mayor’s political heroes. That will be followed by what the new administration is billing as a public block party on a stretch of Broadway known as the “Canyon of Heroes”, famous for its ticker-tape parades.

He now begins one of the most unrelenting jobs in American politics as one of the country’s most-watched politicians.

In addition to being the city's first Muslim mayor, Mamdani is also its first of South Asian descent and the first to be born in Africa. At 34, he is also the city’s youngest mayor in generations.

In a campaign that helped make “affordability” a buzzword across the political spectrum, the democratic socialist promised to bring transformative change with policies intended to lower the cost of living in one of the world’s most expensive cities. His platform included free childcare, free buses, a rent freeze for about one million households, and a pilot of city-run grocery stores.

But he will also have to face other responsibilities: handling trash and snow and rats, while getting blamed for subway delays and potholes.

Mamdani was born in Kampala, Uganda, the son of filmmaker Mira Nair and Mahmood Mamdani, an academic and author. His family moved to New York City when he was seven, with Mamdani growing up in a post-9/11 city where Muslims did not always feel welcome. He became an American citizen in 2018.

Mamdani and his wife, Rama Duwaji, will depart their one-bedroom, rent-stabilised apartment in the outer borough to take up residence in the stately mayoral residence in Manhattan.

He will have to deal with Republican President Donald Trump, who threatened, during the mayoral race, to withhold federal funding from the city if Mamdani won and mused about sending National Guard troops to the city.

But Trump surprised supporters and foes alike by inviting the Democrat to the White House for what ended up being a cordial meeting in November. (AP)