The current study represents the first longitudinal investigation of the potential effects of breastfeeding duration on maternal sensitivity over the following decade. This study also examined whether infant attachment security at 24 months would mediate longitudinal relations between breastfeeding duration and changes in maternal sensitivity over time. Using data from 1,272 families from the National Institute of Child Health and Human Development's Study of Early Child Care and Youth Development, we found that longer breastfeeding duration (assessed up to age 3) predicted increases in observed maternal sensitivity up to child age 11, after accounting for maternal neuroticism, parenting attitudes, ethnicity, maternal years of education, and presence of a romantic partner. Additionally, secure attachment at 24 months was predicted by breastfeeding duration, but it did not act as a mediator of the link from breastfeeding duration to maternal sensitivity in this study. Generating a more specific understanding of how breastfeeding impacts the mother-child dyad beyond infancy will inform recommendations for best practices regarding breastfeeding.