Background Single‐session interventions (SSIs) show promise in the prevention and treatment of youth psychopathology, carrying potential to improve the scalability and accessibility of youth psychological services. However, existing SSIs have conferred greater benefits for youths with anxiety, compared to depression or comorbid problems, and their effects have generally waned over time – particularly for follow‐ups exceeding 3 months. Method To help address these discrepancies, we tested whether a novel SSI teaching growth mindset of personality (the belief that personality is malleable) could reduce depression and anxiety and strengthen perceived control in high‐risk adolescents (N = 96, ages 12–15). At baseline, youths were randomized to receive a 30‐min, computer‐guided growth mindset intervention or a supportive‐therapy control. Youths and parents reported youth anxiety and depressive symptoms, and youths reported their levels of perceived control, at baseline and across a 9‐month follow‐up period. Results Compared to the control program, the mindset intervention led to significantly greater improvements in parent‐reported youth depression (d = .60) and anxiety (d = .28), youth‐reported youth depression (d = .32), and youth‐reported perceived behavioral control (d = .29) by 9‐month follow‐up. Intervention effects were nonsignificant for youth‐reported anxiety, although 9‐month effect sizes reached the small‐to‐medium range (d = .33). Intervention group youths also experienced more rapid improvements in parent‐reported depression, youth‐reported depression, and perceived behavioral control across the follow‐up period, compared to control group youths. Conclusions Findings suggest a promising, scalable SSI for reducing internalizing distress in high‐risk adolescents. Clinical Trial Registration Number: NCT03132298.