Quality mental health care is based on the integration of care across organisations and disciplines. The aims of this study were, first, to assess the extent, characteristics and reported outcomes of publications concerned with youth mental health service integration in Australia and internationally; and second, to investigate the study design quality of evaluative interventions and determine whether the studies report on the cost-effectiveness of the integration in order to inform the reform of youth mental health services by Queensland Health. A systematic search of the peer-reviewed literature and a narrative synthesis were undertaken of English language publications from 21 electronic databases. Inclusion criteria were: published 1998–2014 (inclusive); peer-reviewed research; focused on mental health services integration; reported data for youth aged 12–25 years. The methodological quality of evaluative interventions was assessed using the Quality Assessment Tool for Quantitative Studies developed by the Effective Public Health Practice Project (EPHPP). Twenty-five studies met the inclusion criteria: one (4%) was classified as a measurement research, 13 (52%) as descriptive, and 11 (44%) as interventions including five (45%) evaluative interventions. Four out of the five evaluative interventions reported positive effects of youth mental health service integration. Particular problems included ambiguity of definitions, absence of economic or cost analyses and insufficient consumer involvement. The methodological quality of the interventions was variable with, on average, a moderate level of selection bias and study design. Despite a slight increase in the number of studies in the last couple of years, there are important gaps in the evidence base for youth mental health service integration processes. The relatively small number of evaluative studies and lack of economic evaluations point to the need for additional research in this important area.