The COVID-19 pandemic introduced unique stressors that posed significant threats to adolescent mental health. However, limited research has examined the impact of trauma exposure on vulnerability to subsequent stressor-related mental health outcomes in adolescents. Furthermore, it is unclear whether there are protective factors that promote resilience against the negative impacts of COVID-19 stressors in adolescents with prior trauma exposure. This preregistered study aimed to investigate the impact of trauma on COVID-19 stressor-related mental health difficulties in adolescents, in addition to the role of protective factors. Aims were investigated in a sample of 9696 adolescents (mean age 12.85 ± 0.88 years) from the Adolescent Brain Cognitive Development Study. Linear mixed-effects models were employed to examine (a) the associations of early trauma exposure (exposed v. non-exposed), COVID-19 stressors, and perceived stress, sadness, and positive affect levels during the pandemic period in the US, and (b) the role of protective factors (physical activity, parental support, and improvements in family and peer relationships) in these associations. There was a positive association between COVID-19 stressors and sadness, which was enhanced in trauma-exposed adolescents. Improvements in family and peer relationships mitigated the association between COVID-19 stressors and poor mental health outcomes, regardless of prior traumatic experience. These findings support the hypothesis that prior trauma elevates risk of mental health difficulties in the face of future stressors. Results underscore the protective role of enhanced social relationships as targets for early prevention and intervention in those experiencing acute stressors, regardless of prior traumatic experiences.