Background: Mental health apps have the potential to decrease the burden of disease associated with mental illness. However, few apps have been evaluated for effectiveness. Objective: The aim of this study was to evaluate the effectiveness of the strengths-focused My Coping Plan app in improving mental health and coping. Methods: Participants were 56 Australian University students with self-reported elevated levels of distress (K10 total ≥ 16). Participants were randomly allocated to the intervention (My Coping Plan) or waitlist control condition. Results: At 1-month follow-up, participants in the intervention condition reported significantly lower psychological distress (d = 0.31), improved wellbeing (d = −0.42) and improved healthy coping strategies (d = 0.39) compared with the control condition. There was no significant difference between groups in reported unhealthy coping strategies. The majority of participants downloaded the app and made a coping plan. Just over half used their plan when they were distressed. Conclusions: The My Coping Plan app can improve mental health and wellbeing in the short-term, likely through increased self-efficacy. Professional support may be necessary to support long-term use of healthy coping strategies.