Many with a mental illness have an impoverished everyday life with few meaningful activities and a sedentary lifestyle. The study aim was to evaluate the effectiveness of the 16-week Balancing Everyday Life (BEL) program, compared to care as usual (CAU), for people with mental illness in specialized and community-based psychiatric services. The main outcomes concerned different aspects of subjectively evaluated everyday activities, in terms of the engagement and satisfaction they bring, balance among activities, and activity level. Secondary outcomes pertained to various facets of well-being and functioning. It was hypothesized that those who received the BEL intervention would improve more than the comparison group regarding activity, well-being and functioning outcomes.