Decades of research have documented the distinct ways in which destructive and constructive inter-parental conflict influence family functioning and adolescent development. In particular, parenting quality and emotional security are two processes that are compromised in the context of destructive conflict that are linked with later adjustment problems in adolescents. The present study is an efficacy evaluation of the Family Communication Project (FCP), a randomized controlled trial of a family intervention designed to improve communication and conflict resolution strategies by educating families about the different types of conflict (destructive and constructive) and their implications on emotional security, parenting quality, and adolescent adjustment outcomes. More specifically, the current study investigated program effects on conflict behaviors at post-test, parenting quality and emotional security 6-months post-intervention, and adolescent adjustment outcomes 12-months post-intervention. Participants consisted of N = 225 family triads randomly assigned to one of four conditions: Parent-Adolescent (n = 75), Parent-Only (n = 75), Self-Study (n = 38), or No Treatment (n = 37). Data for the current investigation were drawn from pre-test, post-test, 6-months post-intervention, and 12-months post-intervention. This study tests a series of longitudinal sequential mediation models to examine program effects on long-term adolescent adjustment outcomes. Results revealed that the intervention's effect on internalizing behaviors was mediated by improvements in constructive conflict and sequentially through decreased emotional insecurity. These findings support the utility of a brief family-focused psycho-educational program on improving adjustment outcomes and contribute important insight into the mechanisms of program effects.