Background There is limited evidence for public health policy-makers on the health impacts of urban regeneration programmes. Objectives To assess whether or not the London 2012 Olympic and Paralympic Games, and related urban regeneration, were associated with an increase in physical activity and mental health and well-being; to assess whether or not any benefits were sustained over time; and to capture the experiences of residents of the Olympic host boroughs. Design Quasi-experimental prospective cohort study of adolescents and their parents/carers, with a nested qualitative longitudinal study of families. Setting London boroughs of Newham, Barking and Dagenham, Tower Hamlets and Hackney. Participants A cohort of 2254 adolescents in 25 schools; a repeat cross-sectional study of parents/carers and a sample of 20 families for the qualitative study. Intervention The London 2012 Olympic and Paralympic Games, and urban regeneration primarily associated with the redevelopment of the Olympic Park for legacy use. Primary outcome measures Change in the proportion of respondents meeting physical activity recommendations (using self-reported physical activity); change in the proportion of respondents reporting depression and anxiety and change in well-being score. Main results At 6 months, adolescents who became inactive were less likely to come from the intervention borough (Newham) than from comparison boroughs [risk ratio (RR) = 0.69, 95% confidence interval (CI) 0.51 to 0.93]. At 18 months, there were no statistically significant differences between intervention and comparison boroughs for all adolescent physical activity and screen-time transitions. Those who visited the Olympic Park more than once a month were the least likely to remain inactive (RR 0.11, 95% CI 0.02 to 0.48) and the least likely to become inactive (RR 0.38, 95% CI 0.24 to 0.60) compared with those who were active at baseline and at the 18-month follow-up. No impacts on parental/carer physical activity were observed. Adolescents who were ‘no longer depressed’ (RR 1.53, 95% CI 1.07 to 2.20) or ‘remained depressed’ (RR 1.78, 95% CI 1.12 to 2.83) at 6 months were more likely to be from the intervention borough. For well-being, there was no association between boroughs and change in well-being between baseline and the 6-month follow-up. At 18 months’ follow-up, adolescents who ‘remained depressed’ (RR 1.93, 95% CI 1.01 to 3.70) were more likely to be from the intervention borough than from comparison boroughs. No associations were observed for well-being at 18 months. There was limited evidence of change for parental mental health and well-being. The qualitative study found that residents generally welcomed the unexpected chance to live in a cleaner, safer and more unified environment. The findings suggested that the Games temporarily alleviated certain stressors in the social and physical environment. Overall, the Games lessened participants’ sense of social exclusion and appeared to generate a sense of inclusion and respite, even if this was only temporary. Study limitations include the potential for adolescents to not be assigned the correct level of exposure to urban regeneration and the effect of reductions in central and local public budgets owing to the UK Government’s deficit reduction programme. Conclusions This study provided the highest quality data to date on the short- and medium-term social and health impacts of sporting mega-events. We found limited evidence that the London 2012 Olympic and Paralympic Games had a positive effect on adolescent or parental physical activity, mental health or well-being. Funding The National Institute for Health Research Public Health Research programme.