BACKGROUND: Clinical trials of psychological therapies such as cognitive behavioral therapy typically show sustained posttreatment effects. However, less is known about individuals' outcomes following treatment in routine practice, and additionally, patient representatives have highlighted a need for better postdischarge support. OBJECTIVE: The "Paddle" app aims to address these issues. Paddle allows patients to store their therapy materials, reflect on sessions, and monitor their well-being during and after treatment, completing outcome questionnaires and monthly reviews. METHODS: Study 1 evaluated patients' use of Paddle during treatment in routine practice. Patient and therapist surveys explored feasibility, acceptability, and helpfulness. Study 2 examined the feasibility of using the app post discharge, inviting users to submit monthly questionnaires for 6 months, and seeking feedback via survey. RESULTS: Our findings indicate that Paddle was feasible to implement with few technical problems. Although not all patients wanted to use an app in Study 1, uptake was 66% (111/168) and users found it acceptable and helpful for organizing and remembering therapy information. Most reported using the app on a weekly or fortnightly basis. In Study 2, a total of 321 patients downloaded and used the app at least once, of whom 49% (156/321) submitted follow-up data. Of those who reliably improved during treatment, 73% (86/118) remained so throughout the 6-month follow-up period. Among all users, 20% (31/156) showed further reliable improvement at least once compared to their end-of-treatment score. We introduce the concept of "reliable relapse," which occurred for 36% (32/89) of users who had reliably recovered during treatment, highlighting that some experience fluctuations or deterioration. Feedback highlighted Paddle's value in helping people self-monitor and prioritize their well-being after treatment, with 81% (50/62) suggesting a postdischarge follow-up period longer than 6 months would be helpful. CONCLUSIONS: These preliminary findings suggest that Paddle shows promise in supporting patients to collate therapy resources and monitor their well-being during and after treatment. It may help to improve rates of follow-up data collection, which warrants further investigation.