Background: Pediatric to adult health service (AHS) transition is an inevitable but challenging time for many adolescents and young adults (AYAs) with a chronic illness. As the responsibility of illness management begins to shift from parents to the AYA themselves, many AYAs fail to transition in a timely manner which has important health consequences. Mobile apps have the potential to assist AYAs in self-managing their condition during this vulnerable time, but empirical data on app uptake as well as efficacy with respect to transition outcomes and illness control are lacking. Objective: This pragmatic, single arm pilot trial aimed to evaluate AYA usage of a mobile app called ‘TransitionMate’ (TMApp) which was purpose-built to support chronic illness self-management for youth undergoing transition. Secondary aims were to assess AHS attendance and changes in illness control over the first 12 months post-transition. Upon demonstration of TMApp feasibility (≥70% of participants using TMApp at least once a month), a randomized controlled trial was planned to test efficacy of the app in relation to transition and illness control outcomes. Methods: AYAs aged 16-19 years who were transitioning out of two major pediatric hospitals in Sydney, Australia were recruited. Just prior to transition, participants’ demographic and clinical details were collected and TMApp was installed on their mobile devices. Participants were contacted by telephone at six and 12 months for information on self-reported usage, app usefulness and other feedback. Quantitative cohort-level usage data, including the number of times specific app features were accessed, was tracked via mobile analytics. At 12 months, electronic medical records (eMR) from participants’ designated AHS were accessed for data on AHS attendances, measures of illness control and unplanned hospitalizations. Results: 73 AYAs (30 male; median age 18 years) were recruited, with one withdrawing participation in the first month. Participants were primarily from three chronic illness subgroups: diabetes mellitus (n=23); inflammatory bowel disease (n=19); and cystic fibrosis (n=9). 50% of participants reported using TMApp at one month post-transition. Self-reported usage rates subsequently fell to 25% and 11% at six and 12 months, respectively. Mobile analytics data broadly corroborated self-reported usage. Eight participants who continued to use TMApp until 12 months gave a median usefulness rating of 8/10. Over two-thirds (68%) of participants successfully transitioned to their designated AHS by 12 months. Among the successful transitioners who had illness control data available on eMR, over 80% maintained a stable/improved illness status at six and 12 months. Conclusions: Results showed that TMApp was not used regularly enough by AYAs to demonstrate feasibility and justify progression to a randomized controlled trial. Despite low app uptake, over two-thirds of participants successfully transitioned suggesting that TMApp had minimal influence on transition outcomes. Clinical Trial: Australian New Zealand Clinical Trials Registry ACTRN12620000074998; https://www.anzctr.org.au/Trial/Registration/TrialReview.aspx?id=377349&isReview=true.