Introduction: Physical activity is well recognised as a key risk factor for the management and prevention of mental ill-health. Meta-analytic evidence has shown the leisure-time life domain had the strongest associations with positive mental health. Sport, a sub-domain, of leisure time physical activity has also shown positive associations with increased mental health. Aim: Explore contexts of physical activity with the strongest associations with mental wellbeing in Irish adolescents. Method: A multi-method series of five interlinking studies including a national questionnaire, a non-randomised controlled trial and two rounds of focus group interviews. Results: Study one found a minority of adolescents met WHO’s physical activity guidelines. Frequency of activity were found to decline with age. Frequency of activity was positively correlated with wellbeing and negatively correlated with symptoms of anxiety and depression. Males had higher levels of wellbeing and lower levels of anxiety and depressive symptoms across all sub-groups. Study two found that participating in games twice a week led to significant decreases in symptoms of anxiety and depression. Mental wellbeing significantly increased in those who participated two and three times a week. Increases in mental wellbeing were similar in effect size to increases in self-efficacy. Studies three and four identified five main themes that contribute to and support positive mental wellbeing through physical activity and sport: improvements and achievements; meaningful experiences; setbacks and losses leading to higher resilience; facilitating and maintaining social connections; and opportunities for mindfulness and distraction. Adolescents reporting elevated symptoms of depression increased from 39% to 46% with almost 3 in 5 females reporting symptoms of depression ranging from mild to extreme. There was an 8% reduction in the amount of adolescents who participated in 3 or more sports. No changes in physical activity levels were found overall, despite changes within subgroups and patterns of physical activity. Conclusion: Higher frequencies of physical activity and participation in sports are associated with lower symptoms of anxiety and depression, and higher levels of mental wellbeing. Improvements and achievements from physical activities, meaningful experiences and the psychological skills developed through playing sports appear to be the main contributors to higher levels of wellbeing and may offer ‘protective effects’ against symptoms of anxiety and depression.