To evaluate the effectiveness of Functional Family Therapy (FFT) 42 cases were randomized to FFT and 55 to a waiting-list control group. Minimization procedures controlled the effects of potentially confounding baseline variables. Cases were treated by a team of five therapists who implemented FFT with a moderate degree of fidelity. Rates of clinical recovery were significantly higher in the FFT group than in the control group. Compared to the comparison group, parents in the FFT group reported significantly greater improvement in adolescent problems on the Strengths and Difficulties Questionnaire (SDQ) and both parents and adolescents reported improvements in family adjustment on the Systemic Clinical Outcomes and Routine Evaluation (SCORE). In addition, 93% of youth and families in the treatment condition completed FFT. Improvements shown immediately after treatment were sustained at 3-month follow-up. Results provide a current demonstration of FFT’s effectiveness for youth with behavior problems in community-based settings, expand our understanding of the range of positive outcomes of FFT to include mental health risk and family-defined problem severity and impact, and suggests that it is an effective intervention when implemented in an Irish context.