It is recognised that physical activity has a positive impact on quality of life, social well-being and overall health of people with severe mental illness. However, there is a lack of theory informed programmes that support people with mental illness to adopt regular physical activity behaviour. The aim of this case study was to identify determinants of long-term physical activity among people with severe mental illness that may then inform the development of more suitable physical activity programmes. Semi-structured interviews were conducted with 15 people (13 men and 2 women) with a mean age of 36.7 [standard deviation (SD)=11.8] who had a diagnosed mental illness and were attending a physical activity programme run by a mental health non-governmental organisation. Interview data was analysed using the documentary method to emphasise the perspective of people with severe mental illness. Three participation types were generated in the context of individuals’ physical activity orientation and social background—first ‘rehabilitative orientated’ (physical activity as a supportive measure to re-enter the labour market and develop a daily routine); second ‘social-orientated’ (social well-being within the group as the primary motive); finally, ‘trust-orientated’ (a sense of trust that encourages participation). Based on these type-specific categories, it is suggested that different settings (mental health care centres and sport clubs) might be needed to attract and maintain the physical activity engagement of people with severe mental illness. In the context of sport clubs, it is recommended that coaches undergo training in mental health literacy.