Children in care of the state are amongst the most disadvantaged in society. They have often experienced adverse childhood experiences leading to their care entry including abuse and neglect. Longitudinal data suggests problems children in care of the state experience within adolescence persist into adulthood, showing “a continuing legacy of adversity.” Emerging literature shows that edge of care interventions can bring about benefits. These interventions support families to meet their child’s needs and prevent, or reduce, the likelihood of children going into care. However, it is not clear how or why these interventions work. It is important to develop this understanding to inform the development of effective, theory-informed practice to benefit this population. We reviewed and synthesised published literature to expose mechanisms by which interventions may promote and support family preservation for children at the edge of care. Our synthesis uses a realist approach to examine mechanisms by which interventions, in various contexts, can promote and support family preservation for children at the edge of care. Previous work by the team shaped the initial search strategy and in line with RAMESES realist review guidelines, no restrictions were placed on the types of study to be included in the synthesis. From 7,530 potentially relevant references identified, 61 papers were included in final extraction. Extracted data was themed, prior to developing narrative and formulating programme theories. Effective edge of care service operation seemed to be based on four core programme theories pertaining to the need for family skills training, home-based delivery, dedicated worker, and rapid response to need.