A recent increase in the development of online parent programs calls for the need to understand how effective these strategies are for improving children's mental health. We meta-analyzed the effects of online parent programs on children's behavioral problems. Moreover, we explored the combinations of program components to yield stronger program effects. Medline, PsycINFO, Web of Science, and the Cochrane Library were searched. We included peer-reviewed randomized studies evaluating the effect of an online parent program. Effect sizes (Hedges' g) were calculated from post intervention means and standard deviations. We used Qualitative Comparative Analysis (QCA) to identify pathways to effectiveness and individual content and delivery components that seem sufficient or necessary for yielding high effectiveness. Of 2941 articles, 12 articles with a total of 2025 participants met the inclusion criteria. Online parent programs have significant effects on children's behavioral problems (g = - 0.32; 95% CI, - 0.47 to - 0.17), emotional problems (g = - 0.22; 95% CI, - 0.31 to - 0.13), and parental mental health problems (g = - 0.30; 95% CI, - 0.42 to - 0.17). In the QCA, sending parents reminders to work on the program was the only one sufficient component. In conclusion, online support programs reduce children's behavioral and emotional problems and improve parental mental health. Sending parents reminders to work on the program seems to contribute to high effectiveness. Review Registration This study was registered with PROSPERO, number CRD42017080051.