BackgroundPeople with severe mental illness (SMI) experience higher rates and poorer outcomes of physical long‐term conditions (LTCs). The management of SMI and LTCs is highly complex and many people with SMI rely on informal carers for support, which may lead to high levels of caregiver burden, and caregiver burnout. Caregiver burnout can result in poor health outcomes for informal carers and a reduction in the quality of care they are able to provide. Therefore, it is important to understand the caring experience to identify and address factors that contribute to burden and burnout.MethodsThis paper reports a secondary qualitative analysis of semistructured interviews and focus groups conducted with informal carers of people who have coexisting SMI and LTCs. We recruited 12 informal carers in England between December 2018 and April 2019. The transcripts were coded and analysed thematically.ResultsWe identified two overarching themes and five subthemes. The themes included ‘Fighting on all fronts: Mounting strain between demands and resources’, which described the challenge of providing care in the context of coexisting SMI and LTCs, and ‘Safekeeping: The necessity of chronic hypervigilance’, which captured how informal carers' roles were defined by managing high‐risk situations, leading to hypervigilance and paternalistic approaches to care.ConclusionThe experience of informal carers for people with SMI and coexisting LTCs is marked by limited access to support and the management of significant risk, which could contribute to high caregiver burden. Further primary research is needed to understand how the experiences of the caregiver role for people with SMI and LTCs influence caregiver burden.Patient or Public ContributionOur PPI panel DIAMONDS Voice provided guidance on this study from conception, design and development of interview guides and recruitment materials to final write‐up. DIAMONDS Voice consists of service users and carers who have experience of SMI and LTCs. Three carer members reviewed the final manuscript, and two are credited as authors.