The current study contributes to a sparse literature on moderators of Functional Family Therapy (FFT) by examining whether responsiveness to FFT, measured by a broad range of outcomes, varies by adolescent gender, age, and their interaction. This study was informed by 687 families (n, adolescents = 581; n, caregivers = 933) and utilized a pre–post comparison design. Fixed‐effects regressions with gender, age, and their interaction included as explanatory variables were conducted to calculate the average change in youth mental health, callous–unemotional traits, academic outcomes, substance use, and family functioning. Moderation analyses revealed that according to parent report, girls had significantly greater improvements in peer problems and family functioning, and boys benefited more in increased liking of school. There were differential effects by age, such that older youth had less beneficial mental health outcomes and a smaller decrease in frequency of hash use. The gender by age interaction was significant for adolescents’ report of mental health and family functioning outcomes, which suggests that girls benefit from FFT less than boys during early adolescence, but benefit more than boys in late adolescence. This finding adds to literature which has evidenced that family functioning is particularly important for girls by suggesting that FFT is important for improving older girls’ mental health and family functioning in particular. The study’s results expand the examination of outcomes of FFT to include academic outcomes, and provide insight into key factors that should be considered in addressing adolescent behavioral problems and family functioning.