In this paper we evaluate the effects of free pre-kindergarten (Pre-K) programs on the labor force participation of mothers. We use variation in Pre-K rules across all U.S. states, including income eligibility requirements in some states. To estimate the causal effects of access to pre-K on labor supply we exploit the panel aspect of the monthly Current Population Survey between 2002-2019. Specifically we look at the change in labor market behavior of women when their child becomes age-eligible for Pre-K, controlling for individual factors. We find that access to free Pre-K programs increases overall maternal LFP by 2.3 percentage points. In particular, we find that mothers with the following demographic characteristics significantly increase their labor supply as a result of an access to free Pre-K: married, college educated, White non-Hispanic, residents of metropolitan areas, and those with income either below 200 percent or above 400 percent of the Federal Poverty Level (FPL). Our results are robust across a series of placebo tests and alternative specifications and sample restrictions. (author abstract)