Background The impact of chronic stressors like the COVID‐19 pandemic is likely to be magnified in adolescents with pre‐existing mental health risk, such as attention‐deficit/hyperactivity disorder (ADHD). This study examined changes in and predictors of adolescent mental health from before to during the COVID‐19 pandemic in the Southeastern and Midwestern United States. Methods Participants include 238 adolescents (132 males; ages 15–17; 118 with ADHD). Parents and adolescents provided ratings of mental health symptoms shortly before the COVID‐19 pandemic and in spring and summer 2020. Results Adolescents on average experienced an increase in depression, anxiety, sluggish cognitive tempo, inattentive, and oppositional/defiant symptoms from pre‐COVID‐19 to spring 2020; however, with the exception of inattention, these symptoms decreased from spring to summer 2020. Adolescents with ADHD were more likely than adolescents without ADHD to experience an increase in inattentive, hyperactive/impulsive, and oppositional/defiant symptoms. Adolescents with poorer pre‐COVID‐19 emotion regulation abilities were at‐risk for experiencing increases in all mental health symptoms relative to adolescents with better pre‐COVID‐19 emotion regulation abilities. Interactive risk based on ADHD status and pre‐COVID‐19 emotion regulation abilities was found for inattention and hyperactivity/impulsivity, such that adolescents with ADHD and poor pre‐COVID‐19 emotion regulation displayed the highest symptomatology across timepoints. Lower family income related to increases in inattention but higher family income related to increases in oppositional/defiant symptoms. Conclusions The early observed increases in adolescent mental health symptoms during the COVID‐19 pandemic do not on average appear to be sustained following the lift of stay‐at‐home orders, though studies evaluating mental health across longer periods of time are needed. Emotion dysregulation and ADHD increase risk for sustained negative mental health functioning and highlight the need for interventions for these populations during chronic stressors. Results and clinical implications should be considered within the context of our predominately White, middle class sample.