OBJECTIVE: Sedentary behaviour is associated with poorer mental health in adolescence but no studies have followed participants into mid-life. We investigated the association between after-school sedentary behaviours (screen time and homework) in adolescence with mental wellbeing in adulthood when participants were aged 42. METHODS: Participants (n=2038, 59.2% female) were drawn from The 1970 British Cohort Study (BCS70). At age 16 respondents were asked separate questions about how long they spent in three types of screen based activities (TV, video films, computer games) and homework 'after school yesterday'. Mental well-being and psychological distress were assessed at the age 42 sweep in 2012 using the Warwick-Edinburgh Mental Well-being Scale (WEMWBS) and Malaise Inventory, respectively. RESULTS: After adjustment for all covariates, participants reporting more than 3h of after school screen time as an adolescent had -1.74 (95% CI, -2.65, -0.83) points on the WEMWBS compared with adults reporting less than 1h screen time as an adolescent. Participants that reported high screen time both at age 16 (≥3h/d) and age 42 (≥3h/d TV viewing) demonstrated even lower scores (-2.91; -4.12, -1.69). Homework was unrelated to wellbeing after adjustment for covariates. The longitudinal association between adolescent screen time and adult psychological distress was attenuated to the null after adjustment for covariates. CONCLUSIONS: Screen time in adolescence was inversely associated with mental wellbeing in adulthood.