Although breastfeeding has multiple benefits for baby and mother, including maternal mental well-being, many mothers terminate breastfeeding earlier than they desire. We examined two key factors in breastfeeding duration and maternal mental health––breastfeeding efficacy and family–work conflict. Specifically, we examined the moderating role of family–work conflict in the process of breastfeeding efficacy as a predictor of maternal depression by way of duration. In a sample of first-time mothers, we found that breastfeeding duration mediated the relation between prenatal breastfeeding efficacy and depression at 9 months postpartum for working mothers who experienced low levels of family-to-work conflict. That is, for mothers with low family-to-work conflict, higher expected breastfeeding efficacy during pregnancy predicted a longer duration of breastfeeding, which in turn was associated with lower depression at 9 months postpartum. However, for working mothers with high family-to-work conflict, breastfeeding duration did not emerge as an indirect effect on the relation between efficacy and depression. These findings have important implications for a healthy family–work balance to help new mothers adjust when they return to the workforce and as they transition to parenthood.