IntroductionThe purpose of this study was to evaluate an intervention designed to encourage therapists in a municipality mental health services team to collaborate with their clients when writing journal notes. The team was part of a low‐threshold mental health service for children, adolescents and their parents. The therapists offered individual therapeutic sessions with children, adolescents and parents, as well as family sessions. The therapists documented their encounters in the clients' electronic health records.MethodsThe study used a convergent mixed methods design. First, the quality and quantity of documented therapist–client interactions were measured before and after the intervention using the Scale for the Evaluation of Staff‐Patient Interactions in Progress Notes (SESPI). Cross‐tabulation and logistic regression analyses were used to examine potential differences in the recording of therapists' approaches before and after intervention. Second, we explored the therapists' experiences of participating in this project by conducting individual in‐depth interviews. Qualitative interviews were analysed by systematic text condensation. Finally, the quantitative results from assessing the journal notes were contextualised with the qualitative results from the interviews.ResultsThe number of journal notes describing therapist approach and client response increased by 25% points after the intervention. In the interviews, the therapists acknowledged the intervention's relevance to their practice. However, they invited only a limited number of clients to co‐create journal notes. The clients welcomed the invitation.ConclusionCollaborative documentation challenges established journal documentation practices and may strengthen the client's voice. This study provides suggestions for strengthening the intervention in the future full‐scale studies.