Democrat Zohran Mamdani became New York City's mayor on Thursday, vowing during a public swearing-in ceremony on the steps of City Hall to enact an aggressive agenda aimed at making the nation's largest city more affordable for working people.

Mamdani, a member of his party's left-wing democratic socialist faction, was elected last November in a prominent victory that could influence this year's midterm elections that will determine control of the US Congress. Some Democrats have embraced his style while Republicans portray him as a foil on the national political stage.

The 34-year-old campaigned heavily on cost-of-living issues and was sharply critical of Republican President Donald Trump, whose approval rating has fallen over the past year amid economic concerns.

Many of New York's eight million residents - some with hope, some with trepidation - expect him to be a disruptive political force. In a speech following his public swearing-in, Mamdani promoted core campaign promises of universal childcare, affordable rents and free bus service.

"We will answer to all New Yorkers, not to any billionaire or oligarch who thinks they can buy our democracy," he said. "I was elected as a democratic socialist and I will govern as a democratic socialist."

Crowd chants 'tax the rich'

The program for Mamdani's inauguration included remarks by US Senator Bernie Sanders and US Representative Alexandria Ocasio-Cortez, fellow democratic socialists at the vanguard of the Democratic Party's liberal wing.

Sanders, whom Mamdani calls his inspiration, defended Mamdani's agenda.

"Making sure that people can live in affordable housing is not radical," Sanders said. "It is the right and decent thing to do."

The crowd of several thousand cheered loudly when Sanders called on America's millionaires and billionaires to pay more in taxes, breaking into a chant of "tax the rich."

Even with temperatures well below freezing, the city set up a viewing area along Broadway to allow thousands more to watch a livestream of the ceremony, which included musical performances.

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Mae Hardman-Hill, 27, volunteered for Mamdani's campaign and said it felt like his political momentum was growing.

"I'm a native New Yorker. I've watched the city like you just become less and less affordable, less and less livable," Hardman-Hill said. "I'm really excited for ... regular people to get some power back again."

Prior to the public ceremony, Mamdani was privately sworn in as New York City's mayor in the first minutes of the New Year on Thursday at the historic City Hall subway station, which was decommissioned decades ago and is accessible only a few times a year through guided tours.

Reflecting his Muslim faith, he used the holy Quran for his swearing-in, a first for a New York City mayor.

Republicans blast Mamdani

Hours after Mamdani took office, the lead group tasked with electing Republicans to the US House of Representatives sought to portray him as a "radical socialist."

"Every time Mamdani opens his mouth or swipes his pen, he tattoos the Democrat Party’s failures onto every House Democrat facing voters in 2026," spokesperson Mike Marinella said in a statement that signaled the sizeable role Mamdani is likely to play on the national campaign stage.

Mamdani, a former state lawmaker, promised a freeze on rents and free buses and childcare, building a campaign around affordability issues that some have seen as a path forward for the Democratic Party around the country ahead of November midterm elections.

Dean Fuleihan, the incoming deputy mayor, told the Financial Times that Mamdani will move ahead with plans to increase taxes on millionaires to pay for his campaign promises and balance New York's budget. He added that he doesn't expect rich people to leave the city as a result of potentially higher taxes.

New York Governor Kathy Hochul has said she opposes raising personal income taxes but is considering raising corporate taxes to pay for a potential budget shortfall amid federal cuts.

In one early reversal, Mamdani said on Wednesday that he would no longer seek to end mayoral control of New York City's public school system, the largest in the US, naming veteran educator Kamar Samuels as chancellor.

Mamdani inspired a record-breaking turnout of more than two million voters and took 50% of the vote in November, nearly 10 points ahead of Andrew Cuomo running as an independent and well ahead of Republican Curtis Sliwa.

Inauguration of a new era

The Uganda-born Mamdani has been a sharp critic of Trump on issues such as immigration and said his differences with the president were numerous after a warm White House meeting.

Mamdani raised $2.6 million for the transition and celebrations from nearly 30,000 contributors, more than other mayors on record this century, both by the total and single donations, according to official campaign data that presents disclosures of inaugural expenses beginning with Michael Bloomberg's first term in 2001.

As mayor, Mamdani will move from his one-bedroom Astoria apartment, protected from sharp price hikes by the city rent-stabilization program, to Gracie Mansion, the official residence of New York City mayors on Manhattan's upscale Upper East Side.

Bankers and others in New York, the nation's financial capital, had expressed concern about Mamdani, but since his election many have explored how to work with him.