Background: Postpartum mental disorders are highly prevalent with substantial impact on mother-child bonding and child development. While short-term benefits of an interaction-focused mother-baby treatment for maternal mental health are documented, little is known about the stability of these effects and their influence on child behavioural development. Method: This prospective study included 348 women with postpartum mental disorders who received dyadic treatment at a specialized mother-baby day clinic. Maternal symptoms of depression (EPDS), anxiety (STAI-T), overall psychological distress (BSI-GSI) as well as parenting sense of competence (PSOC) were assessed at admission, discharge, and 1-year follow-up, along with diagnostic classification at admission. At 1-year follow-up, mothers (n = 164) completed the Child Behaviour Checklist (CBCL) to measure child behavioural problems. Results: Maternal psychopathology and PSOC improved significantly from admission to discharge, with clinically meaningful effects. No additional improvements emerged from discharge to 1-year follow-up, except for a tentative reduction in anxiety symptoms. All outcome measures and outcome trajectories regarding anxiety symptoms and overall psychological distress varied by primary clinical diagnosis. Greater maternal symptom improvement from admission to 1-year follow-up was associated with fewer child behavioural problems. However, this effect was not found for symptom changes from admission to discharge when controlling for maternal symptoms at 1-year follow-up. Conclusions: Interaction-focused treatment in a mother-baby day clinic may be associated with clinically meaningful improvements in maternal mental health outcomes up to 1-year follow-up. These long-term improvements may also relate to less child behavioural problems. However, the absence of a waiting list control group warrants cautious interpretation of findings.