The paper describes the development of VoiceIn (enables young people to engage directly with Patient and Public Involvement and Engagement (PPIE) opportunities via a mobile app), using a series of co-design workshops that covered how the initial design of VoiceIn was informed and driven by focus groups with young people, mental health professionals, and PPIE leads, how VoiceIn was refined through collaboration with stakeholders, the priorities for an app to support PPIE, the key features necessary in the PPIE app and the recommended next steps in testing and deploying the digital platform.