ObjectivesIn this study, we seek to explore the relationship between adolescent mental well-being, religion and family activities among a school-based adolescent sample from Northern Ireland.SettingThe Northern Ireland Schools and Wellbeing Study is a cross-sectional study (2014–2016) of pupils in Northern Ireland aged 13–18 years.Participants1618 adolescents from eight schools participated in this study.Outcomes measuresOur primary outcome measure was derived using the Warwick-Edinburgh Mental Wellbeing Scale. We used hierarchical linear regression to explore the independent effects of a range of personal/social factors, including religious affiliation, importance of religion and family activities.ResultsIn fully adjusted models, older adolescents and females reported lower mental well-being scores—for the year-on-year increase in age β=−0.45 (95% CI=−0.84, –0.06), and for females (compared with males) β=−5.25 (95% CI=−6.16, –4.33). More affluent adolescents reported better mental well-being. No significant differences in mental well-being scores across religious groups was found: compared with Catholics, Protestant adolescents recorded β=−0.83 (95% CI=−2.17, 0.51), other religious groups β=−2.44 (95% CI=−5.49, 0.62) and atheist adolescents β=−1.01 (95% CI=−2.60, 0.58). The importance of religion in the adolescents’ lives was also tested: (compared with those for whom it was not important) those for whom it was very important had better mental well-being (β=1.63: 95% CI=0.32, 2.95). Higher levels of family activities were associated with higher mental well-being: each unit increase in family activity produced a 1.45% increase in the mental well-being score (β=0.78: 95% CI=0.67, 0.90).ConclusionsThis study indicates that non-religious adolescents may have lower mental well-being scores when compared with their more religious peers, irrespective of religious denomination. This may relate to both a sense of lack of firm identity and perceived marginalisation. Additionally, adolescents with poor family cohesion are more vulnerable to poor mental well-being.