Approximately 5% of the UK population live with serious mental health problems. Data show that informal caregivers of people with mental illness provide care for the highest number of hours compared to other illness and the economic cost of this care is highest in the UK when compared internationally. People living with serious mental health problems make transitions between different intensities of service as their needs fluctuate, including referral, admission, transfer or discharge. Although caregiving is associated with both stress and positive reward, service transitions are particularly associated with increased stress. This review aimed to investigate what is known about the experiences of informal caregivers during mental health service transitions. An integrative qualitative synthesis was conducted following searches in six bibliographic databases and of the grey literature. Studies published in English between 2001 and 2017 were included if the study focus was on serious mental health problems, the experiences of caregivers and service transitions. Eleven studies were included, appraised using the Mixed Methods Appraisal Tool and synthesised, resulting in four themes: (a) Caregiver information, (b) Caregiver involvement in decisions about care and treatment, (c) Accessing services, (d) Being a caregiver. Caregivers' experiences were similar during transitions to their usual caregiving role but they faced more challenges and their experiences were amplified. Concerns about confidentiality created barriers to information sharing. Continuity of professionals across transitions was helpful. Caregivers struggled to deal with their own conflicting emotions and with the behaviours of the person yet rarely received help. The review findings point to a need for continuity of professionals across service transitions, co-designed and delivered training for professionals and caregivers about information sharing, greater understanding of barriers to implementation of family interventions and interventions that address emotional needs of caregivers.