BACKGROUND Within the UK context, it is unclear whether physical activity and screen time changes between completing compulsory education and the period afterwards, and the factors associated with any change. METHODS A prospective population-based longitudinal design among adolescents (n = 2204 at baseline) was adopted. A self-report questionnaire was administered at baseline (final year of compulsory education) and follow-up (i.e. post compulsory education) to measure physical activity over the previous 7 days and screen time (weekday and weekend) in relation to recommended guidelines. Magnitude of change in physical activity and screen time and key influencing variables associated with changes were analysed. RESULTS For physical activity, there was a significant change in participants meeting guidelines at baseline but not meeting guidelines at follow-up with 81.0% not meeting guidelines at baseline and follow-up. For screen time, there was no significant change between baseline and follow-up, with 70.6% not meeting guidelines at baseline and follow-up. Gender was associated with the change in physical activity with a decline less likely in females. CONCLUSIONS Findings reinforce the importance of reducing physical inactivity and sedentary behaviour during this transition. Factors associated with changes in physical inactivity and sedentary behaviour need further investigation.