This paper assesses the short-run impact of first-year maternal employment on low-income children's cognitive development. The identification strategy exploits an important feature of the U.S.'s welfare work requirement rules – namely, age-of-youngest-child exemptions – as a source of quasi-experimental variation in maternal employment. The 1996 welfare reform law empowered states to exempt adult recipients from the work requirements until the youngest child reaches a certain age. This led to substantial variation in the amount of time that mothers can remain home with a newborn child. I use this variation to estimate local average treatment effects of work-requirement-induced increases in maternal employment. Using a sample of infants from the Birth cohort of the Early Childhood Longitudinal Study, the OLS results show that children of working mothers score higher on a test of cognitive ability. However, the IV estimates reveal sizable negative effects of early maternal employment. An auxiliary analysis of mechanisms finds that working mothers experience an increase in depressive symptoms, and are less likely to breast-feed and read to their children. In addition, such children are exposed to non-parental child care arrangements at a younger age, and they spend more time in these settings throughout the first year of life.