Growing evidence has suggested that parental mental illness and child internalizing and externalizing problems tend to co-occur and engender risk for adverse child outcomes; however, there is considerable heterogeneity in their joint developmental trajectories. This study aimed to evaluate the joint developmental trajectories of maternal and paternal psychological distress and child internalizing and externalizing problems from early childhood to middle adolescence. Given that suicide and self-harm are major public health issues in adolescence and often occur in the context of other mental health issues, we also examined the association between these joint trajectories and these outcomes in adolescence. Parallel-process latent class growth analysis was applied to 14 years of follow-up data from a large-scale, nationally representative sample of youths participating in the UK’s Millennium Cohort Study (MCS; n = 12,520, 50.9% male). Results showed the best-fitting solution had four trajectory classes: (1) low symptoms, 59.0%; (2) moderate symptoms in children, 22.5%; (3) notable symptoms in fathers, 10.7%; and (4) co-occurring maternal and child symptoms, 7.8%. The trajectory groups differed in their self-harm and suicide attempts in adolescence, underscoring the possible importance of the roles of both parental distress and child problem behaviors processes in these outcomes. Future studies will be valuable to rigorously test the directionality and the respective roles of parents and children in this association. Our findings suggest the need for two-generation mental health intervention programs that are tailored based on co-developmental trajectory group membership.Supplementary InformationThe online version contains supplementary material available at 10.1007/s10802-023-01034-3.