BackgroundThe Improving Mental Health Literacy Among Children and Young People in Indonesia (IMPeTUs) intervention is a co-produced, evidence-based digital intervention designed to improve anxiety and depression focused mental health literacy and self-management among people aged 11–15 in Java, Indonesia. This study aimed to evaluate the usability, feasibility and preliminary impact of our intervention.MethodsMixed methods, multi-site case studies based on a theory of change. Pre-and post-assessments of a range of outcomes and qualitative interviews/focus groups with children and young people (CYP), parents and facilitators. The intervention was implemented in 8 health, school and community sites across Java, Indonesia (Megelang, Jakarta and Bogor).Quantitative data designed to understand the impact of and feasibility of evaluating the intervention collected from 78 CYP who used the intervention were analysed descriptively. Qualitative data from interviews and focus groups collected from 56 CYP, 49 parents/caregivers and 18 facilitators were analysed using framework analysis.ResultsQualitative data analysis indicated high levels of usability and acceptability for the interface aesthetic, personalisation, message presentation and navigation. Participants reported minimal burden and no negative outcomes associated with the intervention. CYP, parents and facilitators identified a range of direct and spill over effects of interventions engagement, some of which were not anticipated at study outset. Quantitative data highlighted the feasibility of intervention evaluation, with high levels of recruitment and retention across study time points. Minimal changes were identified in outcomes pre-to-post intervention, which may in part be due to a lack of scale relevance and/or sensitivity to the intervention mechanisms indicated in the qualitative data.ConclusionsDigital mental health literacy applications are potentially an acceptable and feasible way to prevent burdens of common mental health problems amongst CYP in Indonesia. Our intervention and evaluative processes will be further refined prior to definitive evaluation.Supplementary InformationThe online version contains supplementary material available at 10.1186/s13034-023-00608-9.