AbstractGrowing evidence demonstrates that the mental and psychosocial health impacts of caring vary significantly for individual children, depending on who they are, the person that they care for, their responsibilities and the wider family situation. Although individual studies have made progress in identifying the range of impacts, there is a lack of clarity around which impacts affect who and in what circumstances. This synthesis, based on RAMESES realist protocols, aims to increase clarity concerning how and why the mental and psychosocial health impacts of caring for a family member vary for different children. There were 12 391 unique search results screened at title, abstract and full‐paper levels. Forty‐four retained studies were analysed, resulting in the development of a model with 17 context‐mechanism‐outcome configurations. The model divides the configurations into three interlinking domains. The caregiving responsibilities domain considers how the impacts of caring vary with the circumstances of the individual young carer, the person they care for and their family. The identity domain details the development of a caring identity that potentially mitigates the negative effects of caring and enables positive benefits. The support domain concerns the support provided from family, community and services that, depending on quality, can mitigate or exacerbate the impacts of caring. Support also moderates the care identity by affecting self‐perception of the caring role. The model has the potential to inform the development of interventions that target particular mechanisms to enable positive change for young carers. This potential can be enhanced by further research to test the model, with a focus on refining configurations where less evidence is available. There is a particular need to focus on identification which is under‐represented in the model as both a mechanism and a contextual factor due to unidentified young carers being largely absent from past research.