There is growing evidence of a beneficial effect of social group processes on well-being and mental health. To investigate the role of group membership continuity in reducing mental ill-health among young people who were already vulnerable pre-pandemic, and to understand the social and psychological mechanisms of the benefits of group memberships for vulnerable young people. This study takes a cross-sectional design, using survey data from a sample of 105 young people aged 16-35 years, collected approximately 1 year after the global COVID-19 outbreak (January to July 2021). Correlational and path analyses were used to test the associations between group membership continuity and mental health problems (depression, anxiety, psychotic-like experiences) and the mediation of these associations by hope and social connectedness (in-person and online). To correct for multiple testing, the Benjamini-Hochberg procedure was implemented for all analyses. Indirect effects were assessed with coverage of 99% confidence intervals. Multiple prior group memberships were associated with preservation of group memberships during the COVID-19 pandemic. In-person social connectedness, online social connectedness and hope mediated the relationship between group membership continuity and mental health problem symptoms. The results suggest that clinical and public health practice should support vulnerable young people to foster and maintain their social group memberships, hopefulness and perceived sense of social connectedness as means of helping to prevent exacerbation of symptoms and promote recovery of mental health problems, particularly during significant life events.