Apps have the potential to support psychological treatments by providing psychoeducation, increasing homework compliance, and generalizing therapeutic skills outside of sessions. However, there are few apps developed to support specific evidence-based treatment for PTSD for adolescents. This paper shares experiences developing an app to complement Trauma-Focused Cognitive Behavioral Therapy (TF-CBT). It also describes adolescents' and therapists' perceived usefulness and the potential effect of the app 'My everyday life' on posttraumatic stress symptoms (PTSS). We did a needs assessment with adolescents, therapists, and TF-CBT trainers, followed by programming and beta-testing. We carried out a mixed methods study of 59 adolescent patients (aged 13-18 years) and their therapists. Using a triangulation design, we combined quantitative data on app use and changes in PTSS with qualitative data from interviews with 10 adolescents and 10 therapists who described their experiences with the app. The app includes psychoeducation, mental health self-monitoring, coping tools, and goal-setting functionality. The most used features were psychoeducation and breathing exercises. Few therapists actively used the monitoring progress feature. Both adolescents and therapists found the app beneficial for everyday use, though some therapists were unsure about how to integrate it with TF-CBT. Adolescents reported significant PTSS improvement during TF-CBT with the app as a complement. Adolescents with post-treatment data ( = 13) did not report significantly different PTSD improvement compared to adolescents without access to app, but the study lacked power to draw strong conclusions. The app 'My everyday life' can be useful for adolescents receiving TF-CBT. The therapists may benefit from clearer guidelines on how to integrate apps with evidence-based treatments. Digital tools are a promising avenue for supporting the implementation of evidence-based treatment.