Associations between first-year maternal employment and mother—and youth-reported externalizing behavior at age 15 were examined paying attention to potential mediating roles of home and child care environments by 36 months, effortful control at 54 months, and externalizing behavior at 54 months and middle childhood. We used data from the NICHD Study of Early Child Care and Youth Development (n = 922 non-Hispanic White children) which is a prospective birth cohort study that followed children from birth to age 15. Full-time first-year maternal employment was associated with higher levels of externalizing behavior at age 15 through higher maternal depressive symptoms by 36 months, lower effortful control at 54 months, and higher externalizing behavior at 54 months and middle childhood. Part-time first-year maternal employment was not associated with higher externalizing behavior at age 15.