Using the restricted data from American Community Survey from 2011 to 2017, this paper examines the impact of New York City’s (NYC) expansion of universal pre-kindergarten (UPK) on labor force participation of mothers with the youngest child of 4 years of age. Starting in Fall of 2014, any child who is 4 years old and residing in NYC for the past year is eligible for UPK for the academic year, for example all children born in 2010 would qualify for the academic year 2014-15. It uses a triple-difference approach - first compare mothers in NYC with the youngest child of 4-year-olds (treated mothers) to mothers with the youngest child of 5 and 6-year-olds (control mothers) before and after the program. Next, it compares this difference with mothers living in adjacent counties in the New York Metropolitan Area (NMA) in New York to NYC. I find that the program increased mothers’ labor force participation by 5 percentage points (a 7.5 percent impact) in NYC. The results are robust to various robustness checks like comparing with mothers living in all of NMA and mothers in Philadelphia.