This study evaluated feasibility, acceptability, and preliminary efficacy of a 12-week "Guys/Girls Opt for Activities for Life" (GOAL) intervention on 10- to 13-year-old adolescents' body mass index (BMI), percent body fat, physical activity (PA), diet quality, and psychosocial perceptions related to PA and healthy eating. Parent-adolescent dyads from two schools were enrolled. Schools were assigned to either GOAL (38 dyads) or control (43 dyads) condition. The intervention included an after-school club for adolescents 2 days/week, parent-adolescent dyad meeting, and parent Facebook group. Intervention adolescents had greater autonomous motivation for PA and self-efficacy for healthy eating than control adolescents (both p < 0.05). Although between-group differences were not significant, close-to-moderate effect sizes resulted for accelerometer-measured moderate-to-vigorous PA and diet quality measured via 24-hr dietary recall (d = 0.46 and 0.44, respectively). A trivial effect size occurred for percent body fat (d = -0.10). No differences emerged for BMI. Efficacy testing with a larger sample may be warranted.