Background: Mental health is a priority for high schools, but extant programs often focus on symptom reduction rather than promoting adolescents' developmental assets. Objective: The goal of this study is to examine whether Art in Mind (AIM), a strengths-based after-school organized art activity tailored to fit the needs of adolescents with internalizing problems, can promote young people's psychosocial adjustment and support their positive development. Methods: Using a quasi-experimental pre-program, post-program, and 5-month follow-up design, 50 students (mean age = 15.60; 68% female) from four Canadian high schools were assigned to AIM (n = 23) or to a waitlist condition (n = 27). Results: Results from 2 × 2 mixed ANCOVAs revealed significant time by condition interactions reflecting greater improvement in the AIM group with regards to depressive mood at post-program, and to general self-efficacy, social functioning, and positive self-esteem at follow-up. Planned comparisons revealed several significant improvements between pre- and post-program or post-program and follow-up in the AIM group (effect sizes ranging between d = 0.27 and 0.61), while none were observed in the waitlist group. Conclusions: This study supports the potential of after-school organized activities as a strategy to effectively foster positive youth development in adolescents with internalizing symptoms. Further studies are needed to understand the essential implementation parameters and mechanisms of action in these programs.