On Thursday evening, a board he controls made good on that signature pledge.

The city’s Rent Guidelines Board, an independent panel of mayoral appointees, approved a rent freeze covering both one-year and two-year leases for people living in about 1 million rent-stabilized apartments.

The vote, while expected, is a big win for Mamdani, allowing him to beat back skepticism over his ability to deliver on his affordability agenda. And it came just two days after he made a splash as a budding progressive kingmaker when three congressional candidates he endorsed won their races in stunning fashion, unseating two Democratic incumbents and bucking the party establishment.

In a statement, Mamdani, a Democrat, called the decision “a historic victory for New York City tenants.”

“This is the relief that working people across our city deserve,” he said.

Real estate groups say a rent freeze would pinch landlords, leaving them struggling to afford routine maintenance or other repairs as they face rising costs and inflation. Critics of rent regulation say the policy leads to higher rental costs for non-stabilized units.

“This will only result in more dilapidated housing and potentially more foreclosures and bankruptcies, which the city is wholly unprepared for,” said Kenny Burgos, CEO of the New York Apartment Association, a landlord lobbying group.

A legal challenge is expected over the board’s vote. One of its members, an appointee representing landlords, resigned in protest as she claimed the board’s independence had been compromised.

“The Rent Guidelines Board has stopped being a fact-finding body,” former board member Christina Smyth wrote in her resignation letter, which she provided to The Associated Press. “It has become a body that starts with an answer and vibe codes its way backward to justify it.”

Mamdani — who lived in a rent-stabilized apartment in Queens with his wife before moving this year into Gracie Mansion, the city’s stately mayoral residence — appointed a majority of the board’s members a little over a month into his term, signaling a focus on the freeze.

He has said most New Yorkers are desperately in need of relief from high housing costs.

Around 2 million people live in the highly sought-after rent stabilized units, which comprise about 40% of the city’s housing stock. While the apartments are privately owned, the city board votes each year on the maximum allowed rent increase.

The board has frozen rents in the past, most recently under former Mayor Bill de Blasio, but then approved modest increases under Mamdani’s predecessor, former Mayor Eric Adams. Last year, the board approved an increase of up to 3% on one-year leases and up to 4.5% on two-year leases.

This time, however, the board, now stacked with Mamdani appointees, quickly greenlit the freeze, drawing massive cheers from advocates who packed into an auditorium at a museum along Central Park for the vote.

There are no income limits that determine who can live in rent-stabilized units, and it is not uncommon for higher income people to live in such apartments, which has drawn criticism.

During last year’s mayoral race, former Gov. Andrew Cuomo called on Mamdani — then a state Assembly member earning just under a $150,000 salary — to vacate his rent stabilized unit, calling the arrangement “disgusting” as he argued the apartment should go to someone making less money.