People with severe mental illness (SMI) are more likely to experience physical health conditions than the general population. Little is known about the experience of people with SMI using digital health interventions (DHIs) to support their physical health. We explored how people with SMI use DHIs to support their physical health, the acceptability, factors affecting use, and impact on physical health. This was a three-stage mixed methods study (1) online survey of people with SMI; (2) interviews with a subsample of participants from Stage 1; (3) stakeholder workshops. Participants were generally satisfied with the DHIs they used. The most popular DHIs were targeted at diet, exercise, and weight management. Factors that encouraged use included simplicity and data-linkage. Concerns included costs, data security, and reliability of information. Positive impacts included accountability and tangible physical health benefits. Mental health impacted engagement with DHIs. DHIs were seen as a useful tool to monitor physical health but could not replace contact with clinical services. DHIs were considered useful and acceptable by people with SMI and may be used as an extension of clinical care. The specific needs and priorities of people with SMI should be considered both in developing and recommending interventions.