Breastfeeding ≥12 months is recommended for optimal infant nutrition but may hold maternal benefits, as well. Indeed, lactation has been associated with lower long-term risk of diabetes in the mother but the mechanism by which it imparts sustained post-weaning effects on glucose tolerance remains unclear. In this context, we postulated that lactation potentially could induce post-weaning beneficial effects on glucose tolerance by modifying the natural history of insulin sensitivity and/or pancreatic beta-cell function over time. Thus, in this study, we evaluated the relationships between duration of lactation (≤3 months (n=70), 3-12 months (n=140), ≥12 months (n=120)) and trajectories of insulin sensitivity/resistance, beta-cell function, and glycemia over the first 3-years postpartum in a cohort of 330 women comprising the full spectrum of glucose tolerance in pregnancy, who underwent serial metabolic characterization, including oral glucose tolerance tests, at 3-months, 1-year, and 3-years postpartum. The prevalence of dysglycemia (pre-diabetes/diabetes) at 3-years postpartum was lower in women who breastfed ≥12 months (12.5%) than in those who breastfed ≤3 months (21.4%) or for 3-12 months (25.7%)(overall P=0.028). On logistic regression analysis, lactation ≥12 months independently predicted a lower likelihood of pre-diabetes/diabetes at 3-years postpartum (OR=0.37, 95%CI 0.18-0.78, P=0.009). Notably, lactation ≥12 months predicted lesser worsening of insulin sensitivity/resistance (P<0.0001), fasting glucose (P<0.0001), and 2-hour glucose (P=0.011) over 3-years, as compared to lactation ≤3 months, but no differences in beta-cell function (P≥0.37). It thus emerges that adherence to current breastfeeding recommendations reduces future diabetic risk through sustained post-weaning effects on insulin sensitivity/resistance but not beta-cell function.