<p>Mental health interventions delivered through the internet are more scalable, affordable, and demand less training. Yet, the effects of these online programs and factors contributing to the effectiveness, especially for children and adolescents, remain unclear. This meta-analysis aims to not only understand the impact of online mental health interventions for both positive and negative mental health outcomes, but also compare online programs to school-based programs to provide practical implications. The research team applied meticulous search strategies, including database queries, handsearching, forward and backward citation tracking. Double-blinded screening against stringent inclusion criteria identified 28 eligible studies. Overall, online interventions effectively reduced negative outcomes, such as depression and anxiety (<i>ES</i> = 0.19, <i>p</i> &lt; .001, N_ES = 135, 95% PI = [−&#xa0;0.10, 0.53], 95% CI = [0.12, 0.27], I² = 71.4%, τ² = 0.0186). However, no evidence was found for online interventions’ effect on positive outcomes, such as well-being. In the negative outcomes’ model, studies with smaller sample sizes, non-clinical populations, and elementary students produced larger effect sizes. In addition, the analysis compared online interventions targeting depression and anxiety with school-based in-person interventions, based on data collected in a previously published meta-analysis. Results showed that both online and school-based interventions are effective, although in-person programs produced a slightly larger weighted average effect size. Differences between online and in-person randomized controlled trials were not statistically significant. These findings suggest that online mental health interventions can be a viable alternative to supplement in-person interventions, especially for schools and communities with limited resources dedicated to student mental health.</p>