INTRODUCTION: Social media addiction (SMA) and internet addiction (IA) are increasingly prevalent, impacting mental health (MH) globally. This study investigates the mediating roles of mindfulness and social capital (SC) in the relationship between SMA, IA, and MH among Ethiopian high school and university students, contributing to the Sustainable Development Goal (SDG) 3 of good health and well-being. METHODS: A cross-sectional study was conducted with 1160 university and 1473 high school students in Dessie, Ethiopia. Participants completed validated questionnaires assessing SMA, IA, mindfulness, SC, and MH. Structural Equation Modeling (SEM) with a multi- mediation Model (SMM) was used to examine the hypothesized relationships. RESULTS: SEM revealed that both SMA and IA had a direct negative effect on mindfulness, SC, and MH in in both high school and university students. Notably, mindfulness and SC significantly and positively predicted MH, indicating their protective role against the negative effects of SMA and IA. Furthermore, both mindfulness or SC fully or partially mediated the relationship between SMA, IA, and MH, highlighting their crucial role in explaining the association. CONCLUSIONS: This study provides evidence for the crucial roles of mindfulness and SC in buffering the negative effects of SMA and IA on MH among Ethiopian students. The findings highlight the need for educational and health interventions that foster mindfulness and SC to enhance student mental health and promote a healthy digital environment. These results offer valuable insights for educators, health professionals, and policymakers in Ethiopia and other developing countries facing similar challenges.