This study examines adolescents' (N = 28,712; 49% female; M = 12.25, SD = 0.51) recreational screen time and participation in extracurricular activities during after-school hours in association to indicators of positive (optimism, satisfaction with life) and negative (anxiety, depressive symptoms) mental health and wellbeing. Data were drawn from a population-level study with the Middle Years Development Instrument (MDI) with grade 7 students in British Columbia (BC), Canada. The research was implemented in public school districts between 2014 and 2018. We found that adolescents who participated in extracurricular activities (e.g., sports, arts programs, community programs) were significantly less likely to engage in recreational screen-based activities (e.g., watching programs, browsing the internet, playing computer games) for 2 or more hours after school. Findings from Multilevel Structural Equation Modeling analyses showed that extracurricular participation was associated with higher levels of satisfaction with life and optimism, and lower levels of anxiety and depressive symptoms. In contrast, longer screen time (≥2 h/day) was associated with lower levels of satisfaction with life and optimism, and higher levels of anxiety and depressive symptoms; shorter screen time (<2 h/day) was associated with favorable mental health and wellbeing. For screen time, the effect was moderated by gender; the association between longer screen time and poorer mental health and wellbeing was significantly more pronounced for girls than boys. For both boys and girls, mental health and wellbeing were most favorable if they participated in extracurricular activities and reported less than 2 h of recreational screen time per day.