Background: Over 600 RCTs have demonstrated the effectiveness of psychosocial interventions for children and young people's mental health, but little is known about the long-term outcomes. This systematic review sought to establish whether the effects of selective and indicated interventions were sustained at 12 months. Method: We conducted a systematic review and meta-analysis focusing on studies reporting medium term outcomes (12 months after end of intervention). Findings: We identified 138 trials with 12-month follow-up data, yielding 165 comparisons, 99 of which also reported outcomes at end of intervention, yielding 117 comparisons. We found evidence of effect relative to control at end of intervention (K = 115, g = 0.39; 95% CI: 0.30-0.47 I2 = 84.19%, N = 13,982) which was maintained at 12 months (K = 165, g = 0.31, CI: 0.25-0.37, I2 = 77.35%, N = 25,652) across a range of diagnostic groups. We explored the impact of potential moderators on outcome, including modality, format and intensity of intervention, selective or indicated intervention, site of delivery, professional/para-professional and fidelity of delivery. We assessed both risk of study bias and publication bias. Conclusions: Psychosocial interventions provided in a range of settings by professionals and paraprofessionals can deliver lasting benefits. High levels of heterogeneity, moderate to high risk of bias for most studies and evidence of publication bias require caution in interpreting the results. Lack of studies in diagnostic groups such as ADHD and self-harm limit the conclusions that can be drawn. Programmes that increase such interventions' availability are justified by the benefits to children and young people and the decreased likelihood of disorder in adulthood.