With an estimated 90 per cent of the vote counted, Mamdani was ahead with an absolute majority of 50.4 per cent to Cuomo’s 41.6 per cent.

Mamdani was above 50 per cent in Brooklyn, Manhattan and the Bronx (where Cuomo won the primary), and ahead of Cuomo in Queens. Cuomo won the smaller, reliably conservative borough of Staten Island.

The win, though expected based on opinion polls, sets up a mighty battle between Mamdani and Trump, who instructed his followers to vote for Cuomo rather than Republican Curtis Sliwa, who was not a viable candidate and polled 7 per cent.

The Democrats were also celebrating major victories in elections across the US on Tuesday, with moderate candidates Abigail Spanberger and Mikie Sherrill claiming victory as the governors of Virginia and New Jersey respectively, and a Democratic redistricting measure succeeding in California.

In a post on his Truth Social page, Trump blamed the fact that he wasn’t on the ballot himself, as well as the ongoing US federal government shutdown, for the Republicans’ losses, citing unnamed pollsters.

New York congressman Hakeem Jeffries, leader of the Democrats in the House of Representatives, said the results should send a message to the Trump administration and Republicans that they needed to do better on cost of living.

“The American people are saying ‘you have failed us’,” Jeffries said on CNN. “Affordability was on the ballot today and Democrats have won all across the country.”

But New York Republican congresswoman Nicole Malliotakis told CNN: “The story tonight is that the socialist party is taking over the Democrat party … they catered to the radical left. We’re going to learn the hard way in New York City what socialism is.”

Mamdani campaigned chiefly on addressing New York’s affordability problems, particularly for working-class New Yorkers. He plans to build social housing, freeze rents on rent-stabilised apartments, make buses free and introduce universal free childcare for all children under five.

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Mamdani pledged to pay for his promises by levying a 2 per cent city income tax on anyone earning more than $US1 million a year ($1.5 million), and increasing the top corporate tax rate from 7.5 per cent to 11.5 per cent – the same as neighbouring New Jersey.

His critics drew attention to his past support for the “defund the police” movement, and queried his commitment to keeping the city safe. They had also pointed to his remarks about Israel and refusal to condemn the phrase “globalise the intifada”.

Mamdani walked back from those positions in the campaign and said he intended to retain the city’s tough-on-crime police commissioner, Jessica Tisch.

But in his victory speech, he appealed to his left-wing base with a rallying call for diversity, progress and resistance against oligarchy and authoritarianism.

“I am young … I am Muslim, I am a democratic socialist, and most damning of all, I refuse to apologise for any of this,” Mamdani said to cheers.

“New York will remain a city of immigrants, built by immigrants, powered by immigrants – and as of tonight, led by an immigrant.”

Delivering his concession speech, Cuomo cautioned his supporters to stop their enthusiastic booing when he congratulated Mamdani on the win. But he said New York had no tolerance for discrimination and would not abide any behaviour that fanned the flames of antisemitism.

“This campaign was necessary to make that point – a caution flare that we are heading down a dangerous, dangerous road,” Cuomo said. “We made that point, and they heard us, and we will hold them to it.”

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Former Democratic senator and secretary of state Hillary Clinton praised Mamdani for his “inspiring campaign” and said the high voter turnout was a win for democracy.

Former president Barack Obama said the night’s results were a reminder for Democrats that “when we come together around strong, forward-looking leaders who care about the issues that matter, we can win”.

Also marking a decisive win on a ballot measure to redraw the electoral map of California in a way that is advantageous to Democrats, Californian Governor Gavin Newsom said the Democratic Party was “in its ascendancy”.