Family-systems interventions have been proposed as one way of supporting families of people with an intellectual disability (ID) or who are autistic. This systematic review aimed to summarise what family-systems interventions have been studied with this population, what evidence there is for their effectiveness and families' experiences of the interventions. The review was preregistered on PROSPERO (CRD42022297516). We searched five electronic databases, identified 6908 records and screened 72 full texts. Study quality was evaluated using the Mixed Methods Appraisal Tool, and a narrative synthesis was used. We identified 13 eligible articles with 292 participating families. Most studies reported positive effects of the interventions on wellbeing and family relationships, and families reported positive experiences. However, research quality was poor and there are no any sufficiently powered randomised controlled trials demonstrating family-systems interventions' effectiveness for this population. There is a need for higher-quality research to establish whether family-systems interventions are beneficial for families of people who have an ID or who are autistic.