Background Preventing child mental health problems and child obesity have been recognized as public health priorities. The aim of the present study was to examine whether being breastfed (at all or exclusively) in infancy was a predictor of mental well‐being and protective against risk of obesity at age 9. Methods Cross‐sectional data from a large, nationally representative cohort study in the Republic of Ireland was used (n = 8357). Data on breastfeeding were retrospectively recalled. Child mental well‐being was assessed using a parent‐completed Strengths and Difficulties Questionnaire (SDQ). Child's height and weight were measured using scientifically calibrated instruments. Results Logistic regression analyses indicated that, after controlling for a wide range of potential confounding variables, being breastfed in infancy was associated with a 26% (P < 0.05) reduction in the risk of an abnormal SDQ score at 9 years. Being breastfed remained a significant predictor of child mental well‐being when child obesity was controlled for, indicating that being breastfed, independent of child obesity, is a predictor of child mental well‐being. The results of a second logistic regression indicated that, after controlling for a wide range of potential confounding variables, being breastfed for between 11 and 25 weeks was associated with a 36% (P < 0.05) reduction in the risk of obesity at 9 years, while being breastfed for 26 weeks or longer was associated with a 48% (P < 0.01) reduction in the risk of obesity at 9 years. Conclusions Breastfeeding in infancy may protect against both poor mental well‐being and obesity in childhood.