Objective: The aim of this study was to establish the utility of a bespoke mobile app for mental health service users and clinicians. Design: Qualitative design using four focus groups. Methods: Work was conducted in three stages. The first stage involved a focus group with mental health service users and eight clinicians from a mental health early intervention service to discuss the utility of a bespoke mobile app. Visual, verbal and written prompts were used to demonstrate a mobile app and to prompt discussion. The results of the focus groups were used to create a 'visual walk through, non-interactive mock up' mobile app which was used to aid discussion with the same service users and clinicians in stage 2 focus groups. Stage 3 involved development of a mobile app prototype based on focus group feedback. Results: Key ideas emerging from the focus groups were adopted in the design of the app prototype. These were as follows: the use of colour to convey mood; simple mood tracking using familiar trigger icons; a calendar integrated with the service user's care plan; a help button linked to personal support; an avatar to personalise the app; and the inclusion of evidence-based information. Conclusion: Digital health technology is an extremely important asset with scope to improve people's lives when combined with behaviour change techniques. Co-design with service users, clinicians and digital technologists is critical to product design and adoption. The use of quality standard criteria and evidence-based content in app development and evaluation is essential.