Background: Despite various initiatives, the mortality rate in people with severe mental illness increased between 2004 and 2014. Poor health behaviours contribute, including poor diets, physical inactivity, smoking, and excessive alcohol consumption. Evidence is scarce regarding how primary care interventions can influence health behaviour change in people with severe mental illness. We aimed to explore how health behaviours were supported and changed in people with severe mental illness via an intervention (PRIMROSE) informed by the behaviour change wheel. Methods: PRIMROSE was delivered over 6 months in primary care across England. We did a secondary qualitative analysis of semi-structured interviews with staff and patients describing their experiences of PRIMROSE. The sample were selected from a random 20% of primary care practices in England. Topic guides were based on the capability, opportunity, motivation, and behaviour (COM-B) model of behaviour and the Theoretical Domains Framework. We identified how behaviour change techniques were applied by mapping the coded data to the behaviour change technique taxonomy (version 1). We explored barriers, facilitators, and processes of supporting or making health behaviour changes using thematic analysis. Themes were further informed by COM-B. Findings: Between April 27 and Dec 19, 2016, 15 patients and 15 health-care professionals participated from a broad range of backgrounds across England, but with limited ethnic diversity. We identified 20 behaviour change techniques being used. Six factors affected supporting and changing health behaviours: motivation, motivated patients were more proactive to action advice, which was affected by witnessing physical health benefits; health knowledge or perceptions, developing knowledge on how to change health behaviour facilitated motivation but negative health perceptions could be difficult to change; difficulties with patient capability, mental and physical health status affected motivation; staff proficiency, previous experience with mental health and motivational interviewing skills aided supporting patients; social influences, patients felt pressure to achieve goals to avoid staff disappointment—creating healthy social (peer) norms facilitated motivation whereas unhealthy norms prevented changes; and environmental level influences, access to time and resources were perceived as critical. Making use of existing opportunities for behaviour change, such as open spaces for walking, facilitated positive changes. Interpretation: The value of the application of behaviour change techniques and primary health-care professionals in supporting health behaviour change in this population was variable. For some, such techniques helped to increase motivation, capability, and opportunity to change health behaviours. In others, complex challenges that people with severe mental illness experienced prevented behaviour change. Our findings suggest that with training and appropriate resources, primary care health-care professionals can facilitate health behaviour change in some cases, but policy makers and commissioners should consider enabling more interdisciplinary involvement.