This study examines whether program effects on mother-child relationship quality and effective discipline mediated the 6-year longitudinal effects of the New Beginnings Program (NBP) to improve mental health and competence outcomes in 218 adolescents from divorced families in a randomized experimental trial. The NBP is a theory-based and parenting-focused preventive intervention to help children adjust to divorce, and it has previously shown significant main and/or Program x Baseline Risk interaction effects to reduce adolescents' mental health and social adaptation problems and to promote competence. Mediation analyses were conducted using single- and two-group (high and low baseline risk) structural equation modeling. A multiple-methods and multiple-informants approach was used to assess the putative mediators and adolescents' outcomes. Results indicated that program-induced improvement in maternal effective discipline at posttest mediated the intervention effect on adolescents' GPA at the 6-year follow-up. Moreover, program-induced improvement in mother-child relationship quality mediated the intervention effect on adolescents' mental health problems for those with high baseline risk for maladjustment. The discussion focuses on the implications of the mediation findings for advancing the developmental theories that informed the design of the NBP and the implications for implementation of the NBP in community settings. (Contains 2 tables, 5 figures, and 3 footnotes.)