Background Being breastfed is associated with lower cardiovascular risk factors but, to date, no studies have demonstrated a protective effect on cardiovascular disease (CVD). This study aims to address the limitations of previous studies, specifically insufficient statistical power and residual confounding, to determine if such association exists. Methods This is a population-based retrospective cohort study of 320 249 men and women aged 40–69 years. Breastfeeding status was self-reported. CVD and myocardial infarction (MI) events and deaths based via linkage to hospitalization and death records. Results Overall, 28 469 (8.4%) participants experienced a CVD event and 5174 (1.6%) experienced an MI. Following adjustment for sociodemographic, lifestyle and early life confounders, breastfeeding was associated with a reduced risk of CVD events (HR 0.97, 95% CI 0.94–1.00, P = 0.041), CVD deaths (HR 0.91, 95% CI 0.84–0.98, P = 0.017), MI events (HR 0.93, 95% CI 0.87–0.99, P = 0.033) and MI deaths (HR 0.81, 95% CI 0.67–0.98, P = 0.026). Conclusions Child health benefits of breastfeeding are well established. However, the benefits of breastfeeding may extend into later life reinforcing the need to encourage and support breastfeeding.