When young people need health information they are increasingly likely to use online sources and health apps (applications). Yet, these are not necessarily well-designed, reliable or appropriate, and research has primarily focused on adult use. Our study is the first to use qualitative mixed methods (focus groups and interviews) to apply the Technology Acceptance Model (TAM) to understand 26 young people’s uptake and use of a new, clinically-approved health app (application) for 16–25 year olds. We found that perceived usefulness, perceived ease-of-use, social influences and trust, all differently impacted children and young people health-app acceptance and effectiveness. Implications for future research and young-person health-app development are discussed.