Objective: Co-occurring population-level events, such as social unrest and coronavirus disease 2019, are observed in many societies today. Few studies have explored their combined mental health effects on young people. While self-focused rumination has been suggested to be a key mechanism underlying depression, the role of event-based rumination in mediating the impact of population stressors has yet to be elucidated. Methods: Data were collected from 6988 young people in a large-scale community online survey in Hong Kong. The survey assessed symptoms of post-traumatic stress disorder and depression, direct exposure to social unrest-related traumatic events, coronavirus disease 2019 pandemic-related events, personal stressful life events, event-based rumination and other individual risk factors. Results: High levels of comorbid post-traumatic stress disorder and depressive symptoms were observed. Logistic regression analysis revealed that probable post-traumatic stress disorder was associated with traumatic events (odds ratio = 1.73, 95% confidence interval = [1.64, 1.82]), pandemic-related events (odds ratio = 1.08, confidence interval = [1.01, 1.16]), stressful life events (odds ratio = 1.20, confidence interval = [1.21, 1.37]), high event-based rumination (odds ratio = 3.00, confidence interval = [2.58, 3.48]), lower resilience (odds ratio = 1.18, confidence interval = [1.15, 1.21]), higher smartphone reliance (odds ratio = 1.09, confidence interval = [1.05, 1.13]) and financial concerns (odds ratio = 1.25, confidence interval = [1.18, 1.33]). The odds for probable post-traumatic stress disorder was also significantly higher when two or more traumatic events were experienced (odds ratio = 4.03, confidence interval = [3.52, 4.62]). Factors associated with moderate-to-severe level depressive symptoms were similar. Event-based rumination significantly mediated between different types of external events (traumatic events, pandemic-related events, stressful life events) and both post-traumatic stress disorder and depressive symptoms. Conclusion: These findings suggest that diverse types of stressful events during population-level crises could add to personal stressors to affect mental health outcomes in young people. Among other protective and risk factors, event-based rumination presented as a prominent transdiagnostic mediator for different symptom dimensions which may be a potentially important target for early risk detection and intervention.