There is growing interest in the role of organized social participation in clubs, associations and activities in shaping subjective well-being (SWB). However, the field remains contested. This study addresses key questions regarding the participation-SWB link: concerns regarding endogeneity; debate surrounding the mechanisms at work; and the role participation can play in closing inequalities in SWB. Each question is addressed through a quasi-experimental field study into the impact of a large-scale, nationally-implemented youth engagement scheme (UK National Citizen Service) on life satisfaction. Using pre-test/post-test data on a sample of participants and (propensity score matched) controls, results suggest discrete periods of youth engagement can lead to significant improvements in life satisfaction, observable at least 4-6 months after involvement ended. Participation can also help close social inequalities in SWB via a significantly stronger impact on life satisfaction among young people from more economically disadvantaged communities. Although youth from disadvantaged communities join the scheme with lower SWB, post-participation, they have entirely closed the gap in SWB with their less-disadvantaged peers. Improvements in SWB emerge from positive impacts of participation on both social- and psychological-resource pathways. However, stronger participation-effects on psychological-resources become increasingly important for explaining the additional SWB-gains of more disadvantaged young people.