The move came hours before the workers had planned to strike.
Their contract expired with Riverbay at midnight.
The contract covers the 500 porters, handymen, maintenance men, garbage attendants and groundskeepers who maintain the massive complex that is home to 55,000 New Yorkers.
"RiverBay's refusal to put realistic wage proposals on the table was unacceptable, and their lock-out is a blatant disregard for the tenants as well as the workers of Co-Op City." said Kyle Bragg, 32BJ Vice President.
RiverBay rejected the union's offer to extend contract talks, the union said.
The union wants better wages, sick days, vacation days and medical coverage. The owner, RiverBay Corporation, wants workers to switch to a more affordable health care plan.
