Borderline personality disorder (BPD) is characterized by severe disturbances related to understanding oneself and other people and can be reliably detected and treated in adolescence. In this feasibility study, we aimed to focus on the features of, and changes in, narrative identity throughout the course of Mentalization-Based Treatment in Groups (MBT-G) for adolescents with BPD. Six female patients (M= 15.2, SD = 0.75) joined between 16 and 31 (M= 23.83) MBT g sessions. The narrated events within each session across sessions were coded for themes of agency and communion and the narrated reactions were coded for personality functioning. The patients and their parents also completed several self-report measures before and after therapy. Themes of diminished agency and communion were identified, with communion as the dominating theme. When comparing the patients’ first five sessions with their last five sessions, there was an increase in themes related to agency and decreased in communion. The narrated reactions were dominated by themes related to thwarted self-functioning and primarily identity, although intimacy was also present. Patients improved in terms of self-reported functioning and internalizing and externalizing behavior before and after end of treatment. The importance of narration in BPD (group) therapy is discussed alongside clinical implications.