Background: Despite the growing popularity of mobile app interventions, specific engagement components of mobile apps have not been well studied. Methods: The objectives of this scoping review are to determine which components of mobile health intervention apps encouraged or hindered engagement, and examine how studies measured engagement. Results: A PubMed search on March 5, 2020 yielded 239 articles that featured the terms engagement, mobile app/ mobile health, and adult. After applying exclusion criteria, only 54 studies were included in the final analysis. Discussion: Common app components associated with increased engagement included: personalized content/feedback, data visualization, reminders/push notifications, educational information/material, logging/self-monitoring functions, and goal-setting features. On the other hand, social media integration, social forums, poor app navigation, and technical difficulties appeared to contribute to lower engagement rates or decreased usage. Notably, the review revealed a great variability in how engagement with mobile health apps is measured due to lack of established processes. Conclusion: There is a critical need for controlled studies to provide guidelines and standards to help facilitate engagement and its measurement in research and clinical trial work using mobile health intervention apps.