Limited research has explored the trend of adolescent mental health concerns pre- and during the COVID-19 pandemic and how such mental health concerns are explained by emotional resilience and Adverse Childhood Experiences (ACEs). The purpose of this study is to investigate the prevalence and severity of mental health concerns among U.S. adolescents and examine the mediating role of emotional resilience in the relationship between ACEs and mental health concerns (i.e., anxiety and depression). Using the 2018-2021 National Survey of Children's Health, we analyzed a large sample of 12-17 aged adolescents. Descriptive statistics of the prevalence and severity of mental health concerns were conducted. Further, four separate mediation analyses were conducted to examine the association between ACEs, emotional resilience, and mental health concerns for each year. Two key findings are as follows. First, the prevalence and severity of mental health concerns have shown an upward trend across pre- and during COVID-19. Second, all paths in the mediation models were found to be statistically associated with one another over four years. We used parents' reports for variables of interest; further we used a single item to capture emotional resilience. This study not only demonstrates the trend of adolescent mental health concerns before and during COVID-19 but also highlights the crucial role of emotional resilience as a mediator of ACEs and mental health concerns. Findings emphasize the imperative for prevention and intervention efforts to strengthen adolescents' emotional capacity to cope with childhood adversities and additional life challenges.