The landlords say the board's decision last month to freeze rents on rent-stabilized apartments is illegal.
The lawsuit alleges the board is packed with Mamdani loyalists and relied on manipulated data.
In response, the Mayor's office says it's confident the board evaluated all relevant data and that city lawyers are prepared to defend the freeze.
For the first time in its history, New York's Rent Guidelines Board voted to freeze rents for both one-year and two-year leases for the city's one million rent-regulated apartments.
The board voted 7-1 in favor of the rent freeze.