This was to elucidate the experiences and perceptions of people with severe mental illness (SMI) and their health care professionals with the SMILE (Severe Mental Illness Lifestyle Evaluation) group-based lifestyle intervention. SMILE focuses primarily on promoting healthy diet, physical activity and weight loss. A qualitative study with semi-structured interviews was conducted using purposive sampling. Interviews were conducted with 15 clients and 13 health care professionals (HCPs). Data were analysed according to a thematic analysis. Four overall themes were identified: interest in a lifestyle programme; group-based setting; changes in lifestyle behaviour; and preconditions for changing health behaviour. The results showed that clients valued the programme and were interested in the subject of lifestyle. The group-based setting was seen as a positive and important aspect of the intervention. Making lifestyle changes was acknowledged as difficult, especially in combination with the presence of psychiatric symptoms. Clients acquired an improved awareness of different aspects related to lifestyle behaviour. Irrespective of weight loss achieved, clients found their efforts successful with relatively 'small' changes. Some needed more support during the intervention than others. The practical activities in group sessions were regarded as most useful. HCPs were enthusiastic about the programme and their interactions with lifestyle improvements. The results of this study shed light on different aspects that were considered important when delivering a lifestyle intervention to people with SMI. We recommend considering these aspects when implementing a lifestyle intervention in a mental health care setting for clients with SMI.