Background: Children and young people (CYP) with comorbid physical and/or mental health conditions often struggle to receive a timely diagnosis, access specialist mental health care, and more likely to report unmet healthcare needs. Integrated healthcare is an increasingly explored model to support timely access, quality of care and better outcomes for CYP with comorbid conditions. Yet, studies evaluating the effectiveness of integrated care for paediatric populations are scarce. Aim and Methods: This systematic review synthesises and evaluates the evidence for effectiveness and cost-effectiveness of integrated care for CYP in secondary and tertiary healthcare settings. Studies were identified through systematic searches of electronic databases: Medline, Embase, PsychINFO, Child Development and Adolescent Studies, ERIC, ASSIA and British Education Index. Findings: A total of 77 papers describing 67 unique studies met inclusion criteria. The findings suggest that integrated care models, particularly system of care and care coordination, improve access and user experience of care. The results on improving clinical outcomes and acute resource utilisation are mixed, largely due to the heterogeneity of studied interventions and outcome measures used. No definitive conclusion can be drawn on cost-effectiveness since studies focused mainly on costs of service delivery. The majority of studies were rated as weak by the quality appraisal tool used. Conclusions: The evidence of on clinical effectiveness of integrated healthcare models for paediatric populations is limited and of moderate quality. Available evidence is tentatively encouraging, particularly in regard to access and user experience of care. Given the lack of specificity by medical groups, however, the precise model of integration should be undertaken on a best-practice basis taking the specific parameters and contexts of the health and care environment into account. Agreed practical definitions of integrated care and associated key terms, and cost-effectiveness evaluations are a priority for future research.