Rumination and worry are common risk factors of depression and anxiety. Internet-based transdiagnostic interventions targeting individuals with these specific risks may be an effective way to prevent depression and anxiety. This three-arm randomized controlled trial compared the efficacy of Internet-based rumination-focused cognitive behavioral therapy (RFCBT), mindfulness-based intervention (MBI), and psychoeducation (EDU) control among 256 at-risk individuals. Participants' levels of rumination, worry, depressive, and anxiety symptoms were assessed at post-intervention (6 weeks), 3-month, and 9-month follow-ups. Linear mixed model analysis results showed similar levels of improvement in all outcomes across the three conditions. Changes in rumination differed comparing RFCBT and MBI, where a significant reduction in rumination was noted at a 3-month follow-up among participants in RFCBT, and no significant long-term effect among participants in MBI was noted at a 9-month follow-up. All three conditions showed similar reductions in risks and symptoms, implying that the two active interventions were not superior to EDU control. The high attrition at follow-ups suggested a need to exercise caution when interpreting the findings. Future studies should tease apart placebo effect and identify ways to improve adherence.