The rapid growth in the use of online technologies among youth provides an opportunity to increase access to evidence-based mental health resources. The aim of this systematic review is to provide a narrative synthesis of the evidence on the effectiveness of online mental health promotion and prevention interventions for youth aged 12–25 years. Searching a range of electronic databases, 28 studies conducted since 2000 were identified. Eight studies evaluating six mental health promotion interventions and 20 studies evaluating 15 prevention interventions were reviewed. The results from the mental health promotion interventions indicate that there is some evidence that skills-based interventions presented in a module-based format can have a significant impact on adolescent mental health, however, an insufficient number of studies limits this finding. The results from the online prevention interventions indicate the significant positive effect of computerized cognitive behavioral therapy on adolescents’ and emerging adults’ anxiety and depression symptoms. The rates of non-completion were moderate to high across a number of studies. Implementation findings provide some evidence that participant face-to-face and/or web-based support was an important feature in terms of program completion and outcomes. Additional research examining factors affecting exposure, adherence and outcomes is required. The quality of evidence across the studies varied significantly, thus highlighting the need for more rigorous, higher quality evaluations conducted with more diverse samples of youth. Although future research is warranted, this study highlights the potential of online mental health promotion and prevention interventions in promoting youth wellbeing and reducing mental health problems.