BackgroundPeople with developmental disability have higher rates of mental health problems such as anxiety, depression, psychological distress, or a limited sense of belonging to a community. Extracurricular activity can help children and adolescents build social connections beyond family, increasing social capital, which may promote mental health in the transition into adulthood. Little is known about such associations among people with developmental disability. ObjectiveTo examine associations of childhood extracurricular activity with mental health in young adulthood among people with and without developmental disability. MethodsData: Panel Study of Income Dynamics (PSID, 1968–2017), its Child Development Supplement (1997, 2002, 2007) and its Transition into Adulthood Supplement (2005–2019) (n = 2801). Time diaries measured time in activity. Outcomes were psychological distress (Kessler K6) and flourishing (Mental Health Continuum-Short Form). Adjusted linear regressions modeled associations. ResultsIn nationally representative results, 9.6 % (95 % confidence interval, CI 7.8, 11.4) had a disability. Children without disability reported more average weekly time in group activity, 125.1 min (CI 113.2, 136.9) vs. 93.6 (CI 55.1, 132.0; not significant at conventional levels). In adjusted results, “some” group activity (0–180 weekly minutes) was associated with greater flourishing for those with developmental disability (0.89; CI 0.16, 1.61). ConclusionsAmong people with developmental disability, group activity in childhood was associated with greater flourishing in young adulthood. More research is needed to understand the complex nature of activity participation for children with developmental disabilities.