Objective: To examine the impact of a self-determination theory-informed school-based lunchtime games intervention on wellbeing and symptoms of anxiety and depression among adolescent girls in Ireland. Design: Non-randomised controlled intervention design. Method: This self-determination theory-informed intervention consisted of lunchtime games for girls aged 13 ± 0.7 years, 3 days a week. After a 2-week 'sampling' period, students self-selected to 0, 1, 2 or 3 days of lunchtime games. Intervention and control groups were compared by pre- and post-testing. Participants were included in the intervention design via a survey to ascertain the most popular games and activities at the outset. Results: Participating in games twice a week led to significant decreases (p < 0.01) in symptoms of anxiety and depression while the control group showed significant increases (p < 0.01) in symptoms of anxiety and depression. Mental wellbeing significantly (p < 0.01) increased among those who participated two and three times a week. Increases in mental wellbeing were similar in effect size to increases in self-efficacy. Conclusion: Higher frequencies of physical activity led to increased mental health. Self-efficacy may act as an underlying mechanism for increases in mental health through physical activity.