Understanding the mechanisms of change of digital therapeutics is a critical step to improve digital health outcomes and optimize their development. Access to and engagement with digital content is arguably a core mechanism of change of these interventions. However, the mediational role of app engagement has been largely unexamined. To evaluate the mediational effect of engaging with a digital therapeutic for smoking cessation designed for adults with psychiatric disorders. Secondary analysis of a pilot clinical trial of 62 adults with serious mental illness who were randomized to receive either a tailored digital therapeutic (Learn to Quit) or a digital therapeutic for the general public (NCI QuitGuide). Engagement was captured using background analytics of app utilization, including (a) number of interactions with app content, (b) minutes/day of app use, and (c) number of days used. The main outcome was reductions in cigarettes per day from baseline to the four-month endpoint. Mediational analysis followed the Preacher and Hayes bootstrap method. Number of application interactions fully mediated reductions in cigarettes per day in the Learn to Quit application but not in QuitGuide (Average Causal Mediation Effect = .31, p = .02). Minutes/day of app use played an uncertain role, and number of days used was not a significant mediator. Results suggest that one of the mechanisms of action of the Learn to Quit device, engagement with theory-based content, functioned as intended. Future research of digital therapeutics should emphasize granular approaches to evaluating apps' mechanisms of action.