A New York City panel voted on Thursday to freeze rents for nearly one million rent-stabilized apartments, fulfilling one of Mayor Zohran Mamdani’s key campaign promises and handing the mayor a major policy win six months into his tenure.

The panel, known as the Rent Guidelines Board, approved the freeze on both one- and two-year leases in a 7-to-1 vote, pausing increases on more than 40 percent of all apartments across the five boroughs — a mix that includes high-rise luxury apartments, deeply affordable subsidized units and 150-year-old walk-ups. New York City now may offer a model for rent control efforts in other cities and states.

The freeze would apply to leases beginning on or after Oct. 1.

The decision on a contentious issue adds to an already triumphant week for the mayor, who enjoyed a stunning political victory on Tuesday when three left-leaning candidates he endorsed won congressional primaries.

The vote on Thursday took place at El Museo del Barrio, a museum in East Harlem. Dozens of pro-tenant activists flooded an auditorium, holding signs and chanting.

The outcome reflected Mr. Mamdani’s view that an aggressive public sector should work to rein in private industry. But it also reinforced a view of housing policy that is growing in popularity on the left: that strict rent regulation should be paired with unleashing more private development to help ease the city’s housing shortage.

The real estate industry had vociferously opposed a rent freeze, saying it would hurt landlords’ ability to properly maintain their buildings. Landlords and even some independent housing experts have been raising alarms in recent years about the increasing financial burden of insurance, maintenance, taxes and other costs.

The board’s annual vote takes into account those concerns, in addition to factors affecting renters, like incomes and unemployment rates.

The panel includes two members representing landlords, two representing tenants and five “public” members, including one who serves as the chair. It is supposed to operate independently, though Mr. Mamdani appointed a majority of its members.

On Thursday morning, one of the board’s members who represents landlords, Christina Smyth, resigned, saying she felt the vote was predetermined. That prompted an extraordinary public statement from the board’s chair, Chantella Mitchell, saying that she wanted to “to affirm the independence with which this year’s board members have served.”

The cost and availability of housing has been a key part of the mayor’s affordability agenda. New York’s current vacancy rate is just 1.4 percent, according to the most recent city survey — so low that the city has called it an emergency.

Though the yearly vote has been a flashpoint in New York City housing politics for decades, the debate has been particularly fraught this year because of Mr. Mamdani’s campaign promise, which he announced a year ago in a video on social media, saying he would appoint only members who “understood landlords are doing just fine.”

By February, he had appointed six of the members now serving on the board, including Ms. Mitchell.

Over the next few months, the board held a series of public hearings, considered testimony from tenant advocates and landlords and pored over research on housing and the economy.

Mr. Mamdani and his supporters have noted that the board has frozen rents on one-year leases a few times before, including in 2015 (the first time in its 46-year history), 2016, 2020 and the first half of 2021.

But in two of those years — 2015 and 2016 — landlords still had the ability to raise rents through other mechanisms, including when an apartment was vacated. Those mechanisms were eliminated by a series of pro-tenant laws passed by the state in 2019.

That legislation is one reason revenues have been declining for landlords like the Bronx Pro Group, a for-profit developer and owner of affordable housing that has about 3,700 rent-stabilized apartments in its portfolio.

Those apartments are all subsidized or financed by public agencies in some way, said Samantha Magistro, the chief executive of Bronx Pro, and many have razor-thin margins.

Ms. Magistro, who is also the board chair of the New York State Association for Affordable Housing, an advocacy group for the affordable housing industry, said the cost of insurance went up 60 percent between 2019 and 2022, and her company’s overall expenses grew 40 percent between 2022 and 2025.

That is in part because of general inflation and the increasing age of buildings, she said. At the same time, increases permitted by the Rent Guidelines Board haven’t kept pace.

Ms. Magistro said that now about half of her company’s portfolio either just breaks even or operates at a deficit. That has meant reducing the scope of renovations — patching stoves and refrigerators instead of replacing them with new ones, for example.

Ms. Magistro acknowledged the need and benefits of helping people afford to live in the city. But she said the overall trend for landlords was troubling.

“The city’s really going to have to get creative on how to help these underwater properties,” she said. “I don’t know how long they can keep going in this direction.”