Mothers' depressive symptoms are associated with their children's internalizing and externalizing behavioural problems. Because mothers' depressive symptoms can vary across the prenatal and postnatal periods, considering their trajectories is important when predicting children's behavioural problems. The purposes of this study were to: (1) identify profiles of mothers characterized by their prenatal and postnatal (up to 3 years postpartum) depressive symptom trajectories and (2) examine the associations between maternal depressive symptom profile trajectories and preschool children's internalizing and externalizing behavioural problems at 5 years of age. This study used data derived from the APrON Study. The Edinburgh Postnatal Depression Scale measured mothers' depressive symptoms in early (<27 weeks) and late (≥27 weeks) pregnancy and at 3, 6, 12, 24, and 36 months postpartum. The Behavioural Assessment Scales for Children, 2nd Edition, quantified children's internalizing and externalizing problems at approximately 60 months of age. Non-growth latent profile analysis determined the most suitable and parsimonious number of maternal depressive symptom profiles, and linear regression analysis quantified their associations with their 5-year-old-children's behavioural problems. A three-profile structure characterized maternal depressive symptom trajectories: minimal, subclinical, and high. Unadjusted (n = 704) and adjusted (n = 621) analyses showed that: 1) mothers' subclinical and high depressive symptom profiles ( < 0.01) predicted children's internalizing problems and 2) mothers' subclinical depressive symptom profiles ( < 0.01) predicted externalizing problems. Maternal subclinical depressive symptoms were equally, if not more, important compared to high depressive symptoms in predicting children's behavioural problems. Overlooking mothers with subclinical depressive symptoms could have implications for their children's behavioural/mental health.