Adolescent borderline personality disorder (BPD) is a devastating disorder, and it is essential to identify and treat the disorder in its early course. A total of 34 female Danish adolescents between 15 and 18 years old participated in 1 year of structured mentalization-based group therapy. Twenty-five adolescents completed the study, of which the majority (23) displayed improvement regarding borderline symptoms, depression, self-harm, peer-attachment, parent-attachment, mentalizing, and general psychopathology. Enhanced trust in peers and parents in combination with improved mentalizing capacity was associated with greater decline in borderline symptoms, thereby pointing to a candidate mechanism responsible for the efficacy of the treatment. The current study provides a promising rationale for the further development and evaluation of group-format mentalization-based treatment for adolescents with borderline traits.