Objectives: The co-occurrence of different classes of population-level stressors, such as social unrest and public health crises, is common in contemporary societies. Yet, few studies explored their combined mental health impact. The aim of this study was to examine the impact of repeated exposure to social unrest-related traumatic events (TEs), coronavirus disease 2019 (COVID-19) pandemic-related events (PEs), and stressful life events (SLEs) on post-traumatic stress disorder (PTSD) and depressive symptoms, and the potential mediating role of event-based rumination (rumination of TEs-related anger, injustice, guilt, and insecurity) between TEs and PTSD symptoms. Methods: Community members in Hong Kong who had utilized a screening tool for PTSD and depressive symptoms were invited to complete a survey on exposure to stressful events and event-based rumination. Results: A total of 10,110 individuals completed the survey. Hierarchical regression analysis showed that rumination, TEs, and SLEs were among the significant predictors for PTSD symptoms (all P < 0.001), accounting for 32% of the variance. For depression, rumination, SLEs, and PEs were among the significant predictors (all P < 0.001), explaining 24.9% of the variance. Two-way analysis of variance of different recent and prior TEs showed significant dose-effect relationships. The effect of recent TEs on PTSD symptoms was potentiated by prior TEs ( P = 0.005). COVID-19 PEs and prior TEs additively contributed to PTSD symptoms, with no significant interaction ( P = 0.94). Meanwhile, recent TEs were also potentiated by SLEs ( P = 0.002). The effects of TEs on PTSD symptoms were mediated by rumination (β = 0.38, standard error = 0.01, 95% confidence interval: 0.36 to 0.41), with 40.4% of the total effect explained. All 4 rumination subtypes were significant mediators. Conclusions: Prior and ongoing TEs, PEs, and SLEs cumulatively exacerbated PTSD and depressive symptoms. The role of event-based rumination and their interventions should be prioritized for future research.