Background: This study investigates if a non-randomized controlled perinatal health intervention which offers (a) long-term midwife-led breastfeeding support and (b) psychosocial support of women, is associated with the initiation, exclusivity and duration of breastfeeding. Methods: A sample of 1080 women who attended a 12-month intervention before and after childbirth, during a five-year period (January 2014–January 2019) in a primary mental health care setting in Greece, was examined. Multiple analyses of variance and logistic regression analysis were conducted. Results: The vast majority of women (96.3%) initiated either exclusive breastfeeding (only breast milk) (70.7%) or any breastfeeding (with or without formula or other type of food/drink) (25.6%). At the end of the 6th month postpartum, almost half of the women (44.3%) breastfed exclusively. A greater (quantitatively) midwifery-led support to mothers seemed to correlate with increased chance of exclusive breastfeeding at the end of the 6th month postpartum (p = 0.034), and with longer any breastfeeding duration (p = 0.015). The absence of pathological mental health symptoms and of need for receiving long-term psychotherapy were associated with the longer duration of any breastfeeding (p = 0.029 and p = 0.013 respectively). Conclusions: Continuous long-term midwife-led education and support, and maternal mental well-being are associated with increased exclusive and any breastfeeding duration.