BACKGROUND: Given the prevalence and recent increases in youth mental health problems, there is a pressing need for interventions that target transdiagnostic protective factors that could be targeted as mechanisms of change in treatment. Such interventions are most likely to succeed in meeting population needs if they are scalable, sustainable, and effective. METHODS: We examined whether parent-youth attachment anxiety and avoidance predicted reductions in internalizing and externalizing problems in a large one-group clinical sample of youth ( RESULTS: Findings confirmed that parent and youth reports of attachment anxiety and avoidance, as well as internalizing and externalizing problems, significantly declined over the course of the intervention. Parent reported reductions in youth attachment anxiety, but not avoidance, predicted declining levels of youth internalizing problems. As well, parent reported reductions in youth attachment avoidance and anxiety predicted declining youth externalizing behavior. In contrast, youth reports of reductions in youth attachment anxiety, but not attachment avoidance, were associated with declines in youth externalizing problems. CONCLUSION: Our findings support the role of attachment as an important transdiagnostic mechanism of change in attachment-based programs for parents of teens with clinically significant mental health problems.