Background There is scarce and conflicting evidence on medium‐ to long‐term effects of prolonged breastfeeding on child behavior. Method A population‐based birth cohort study started in 2004 in the city of Pelotas, Southern Brazil. Children were followed up at 3, 12, 24, and 48 months and 6 years of age. Breastfeeding duration was determined based on information collected around the time of weaning. Psychiatric disorders were assessed using the Development and Well‐Being Assessment (DAWBA). Children who were never breastfed were excluded from the analysis. Crude and adjusted analyses were performed using Poisson regression with robust variance. Results Data on breastfeeding and mental health at the age of 6 years were available for 3377 children. Prevalence of breastfeeding for 24–35 months and ≥36 months was 16.1% (95% CI: 14.8–17.3) and 8.1% (95% CI: 7.2–9.1), respectively. Prevalence of psychiatric disorders among those who were breastfed for <24 months, 24–35 months and ≥36 months was 12.4% (95% CI: 11.1–13.7), 13.1% (95% CI: 10.4–16.2) and 12.3% (95% CI: 8.7–16.8), respectively. No association was found between breastfeeding for 24 months or more and psychiatric disorders among children aged 6 years both in the crude and adjusted analyses. Conclusions In this cohort there was no association between breastfeeding for 24 months or more and an increased prevalence of psychiatric disorders at the age of 6 years. Studies analyzing the medium‐ and long‐term effects of prolonged breastfeeding for 2 years or more are scarce and further research is needed regarding this practice.