Billionaire hedge fund manager Bill Ackman is sounding the alarm that Mayor Zohran Mamdani's economic policies, rent freezes, city-run grocery stores, and a purge of private-sector advisers, will drive wealth and jobs out of New York City.

Ackman laid out his case in a Fortune interview published Wednesday, warning bluntly that the mayor's proposals would worsen the city's housing shortage, chase away top taxpayers, and punish the very investment that keeps New York's economy afloat. The broadside came in the same week Mamdani dismissed the entire advisory board of the Mayor's Fund to Advance New York City, a nonprofit that has raised more than $100 million for the families of rescue workers killed or injured on September 11, 2001, and $54.5 million during the COVID pandemic.

Ackman, one of Wall Street's most prominent conservative voices, did not hold back about where the mayor's agenda leads.

"Watch what happens to New York City if Mamdani succeeds in implementing these plans."

Rent freeze would pull thousands more apartments off the market, Ackman argues

At the center of Ackman's critique is Mamdani's proposal to freeze rents on nearly one million stabilized apartments across the city. Ackman told Fortune that the plan would make an already dire housing crunch worse by discouraging landlords from renovating and maintaining units, because they cannot recover the costs under existing rent regulations.

He pointed to a figure he said illustrates the damage already done.

"That's what happened in New York, about 60,000 units are off the market because renovation costs can't be recovered under rent regulations."

Ackman framed the problem as straightforward supply-and-demand economics. Housing costs in New York are already punishing, he said, because left-leaning city leaders have made it extremely difficult for developers to build.

"[Housing costs are] so high because left-wing mayors have made it very difficult for developers to build here, and Mamdani, by freezing rents, is just going to make the problem worse."

A rent freeze may sound like relief to tenants struggling with rising costs. But Ackman's argument, and one that housing economists across the political spectrum have echoed for decades, is that price controls shrink the supply of available units, leaving fewer apartments on the market and pushing rents even higher for everyone not lucky enough to hold a stabilized lease. The millionaire exodus already draining billions in tax revenue from New York only compounds the fiscal risk.

Mamdani's social media stunt rattled Citadel's Ken Griffin

Ackman also zeroed in on Mamdani's confrontational posture toward the city's wealthiest residents. The mayor recently filmed a social media video touting the newly enacted pied-à-terre tax, a levy on high-end second homes, while standing in front of Citadel CEO Ken Griffin's Central Park South penthouse. Griffin, who Ackman said spent $250 million on the apartment, publicly clashed with Mamdani over the move and threatened to reduce his investment in New York.

Griffin called the video a "personal attack" and a "profound lack of judgment." Ackman told Fortune that antagonizing someone like Griffin is self-defeating for a city that depends on high earners to fund public services.

"You want people like Ken Griffin locating Citadel here, spending $250 million on an apartment, because that purchase makes a building economically viable, which creates construction jobs and brings in wealthy residents who pay taxes."

Ackman extended the logic beyond Griffin. He warned against policies that would discourage figures like Elon Musk from locating businesses in the city.

"You want the top earners, who are a small share of the population, paying a large share of taxes... But you don't want to discourage people like Elon Musk from locating their businesses here."

Not every business leader is heading for the exits. Earlier in the week, the CEO of Vornado Realty Trust announced that the company will move forward with plans to build a $4.5 billion skyscraper at 350 Park Avenue that will house Citadel's local offices. But Ackman's broader point is that one project continuing does not erase the chilling effect of a mayor who singles out individual business leaders on social media and pursues policies designed to punish wealth creation.

Advisory board purge strips the Mayor's Fund of private-sector expertise

Mamdani's decision this week to dismiss the entire advisory board of the Mayor's Fund to Advance New York City sent its own signal. The board included executives tied to Blackstone, Citigroup, the Real Estate Board of New York, and several major real estate firms, the kind of private-sector leaders whose relationships have historically powered the fund's ability to raise money for city programs.

City Hall said the mayor plans to install a new board that will put people "directly impacted" by city policies in the rooms where decisions are made. The framing is familiar: replace the people who built the institutions with people who consume their services, and call it equity.

The fund's track record tells a different story about what those private-sector relationships produced. More than $100 million for the families of 9/11 first responders. Another $54.5 million during the pandemic. Mamdani swept all of that institutional knowledge off the board in a single move, with no public explanation of what the replacement board will look like or how it will raise comparable sums.

The pattern is consistent. Mamdani has proposed city-owned grocery stores, inserting government into retail food markets, alongside the rent freeze and the pied-à-terre tax. Each policy expands the reach of city government into sectors where private enterprise has historically operated, and each carries the risk of discouraging the investment and expertise the city needs to function.

Ackman is not the only prominent figure raising the alarm. JPMorgan Chase CEO Jamie Dimon called Mamdani "more a Marxist than a socialist" and dismissed his policies as "ideological mush." Former Mayor Rudy Giuliani was blunter: "I think he's going to destroy New York City. I think this is a disaster time for New York City. This guy becomes mayor, the city is gone."

Opposition spans both parties, and Ackman is putting money behind it

The resistance forming against Mamdani's agenda is unusually broad. Republicans, Democrats, independents, and billionaires are mobilizing against a mayor who won the Democratic primary by 12 points over Andrew Cuomo. Ackman himself is backing a political action committee called "New Yorkers for a Better Future Mayor 25," which launched with at least $20 million to oppose Mamdani. Bo Dietl and Giuliani are attempting to raise $10 million through a separate outside group.

That a Democratic primary winner would face this level of organized opposition from within his own city's establishment, including prominent Democrats, reflects just how far outside the mainstream Mamdani's platform sits. The wave of Mamdani-backed socialists routing Democratic incumbents in New York has rattled party leaders well beyond City Hall.

Ackman, for his part, acknowledged in his interview that Mamdani is "very smart, very charismatic, very good at politics." The criticism is not about the mayor's abilities. It is about what he intends to do with them. Ackman's argument is that socialism, no matter how skillfully marketed, produces the same results everywhere it is tried: scarcity, flight, and decline.

Even Senator John Fetterman has sounded the alarm about the Democratic Party's socialist surge after the New York primary upsets, a sign that the concern extends well beyond Wall Street boardrooms.

The New York Post reported it has sought comment from Mamdani on Ackman's criticisms. No response was included. That silence may be strategic, or it may reflect a mayor who believes his base does not care what hedge fund managers think about rent policy.

But Ackman's warning is not really aimed at Mamdani. It is aimed at the voters, taxpayers, and small business owners who will live with the consequences if these policies take full effect. Rent freezes that shrink the housing supply. City-run grocery stores that compete with private retailers using taxpayer money. A mayor who surrounds himself with loyalists while purging the experienced advisers who helped the city raise hundreds of millions for its most vulnerable residents.

New York has survived bad policy before. But every city has a threshold, a point where the people who fund the tax base, build the buildings, and create the jobs decide the hassle is no longer worth it. Ackman is betting Mamdani's agenda will find that line. The question is whether New Yorkers will recognize it before the damage is done.