Objective Despite substantial evidence of the benefits of breastfeeding for both mothers and children, rates of sustained breastfeeding in the United States are quite low. This study examined whether mandated coverage of lactation support services under the Affordable Care Act (ACA) affects breastfeeding behavior. Data Source We studied the census of U.S. births included in the National Vital Statistics System from 2009 to 2014. Study Design We used regression‐adjusted difference‐in‐differences (DD) to examine changes in breastfeeding rates for privately insured mothers relative to those covered by Medicaid. We adjusted for several health and sociodemographic measures. We also examined the extent to which the effect varied across vulnerable populations—by race/ethnicity, maternal education, WIC status, and mode of delivery. Principal Findings Results suggest that the ACA mandate increased the probability of breastfeeding initiation by 2.5 percentage points, which translates into about 47,000 more infants for whom breastfeeding was initiated in 2014. We find larger effects for black, less educated, and unmarried mothers. Conclusions The Affordable Care Act–mandated coverage of lactation services increased breastfeeding initiation among privately insured mothers relative to mothers covered by Medicaid. The magnitude of the effect size varied with some evidence of certain groups being more likely to increase breastfeeding rates.