The current longitudinal study examined a participation-based intervention for homeless and formerly homeless clients, growth through participation (GTP), developed by a Dutch organization providing shelter services and ambulatory care. GTP is based on a combination of group and individual approaches, whereby clients are enabled to learn to identify their strengths and talents, to develop social skills through interaction with each other, and to learn to once more lead a structured life. The study was conducted among 172 (formerly) homeless clients and comprised three measurement time points. It examined whether (1) quality of life increased during the GTP intervention; (2) social participation (e.g., labor/recreation), self-esteem, clients’ experiences with care (i.e., satisfaction with the services received and with the client–worker relationship), and psychological distress improved during GTP; (3) clients exhibiting psychological distress benefit more from GTP than others. Results from latent growth modeling showed that quality of life and the amount of time clients spent on labor activities increased significantly, but the amount of time clients spent on recreational activities decreased over time. Clients with psychological distress experienced increased quality of life and self-esteem, and reduced psychological distress. Other variables did not significantly change during GTP. Although not all hypotheses were (fully) confirmed, it can be concluded that GTP seems to be a potentially promising intervention. It is recommendable to conduct a multisite RCT to determine the efficacy of GTP.