Background People with personality disorder experience long waiting times for access to psychological treatments, resulting from a limited availability of long-term psychotherapies and a paucity of evidence-based brief interventions. Mentalisation-based treatment (MBT) is an efficacious therapeutic modality for personality disorder, but little is known about its viability as a short-term treatment. Aims We aimed to evaluate mental health, client satisfaction and psychological functioning outcomes before and after a 10-week group MBT programme as part of a stepped-care out-patient personality disorder service. Method We examined routinely collected pre–post treatment outcomes from 176 individuals (73% female) aged 20–63 years, attending a dedicated out-patient personality disorder service, who completed MBT treatment. Participants completed assessments examining mentalising capacity, client satisfaction, emotional reactivity, psychiatric symptom distress and social functioning. Results Post-MBT outcomes suggested increased mentalising capacity (mean difference 5.1, 95% CI 3.4–6.8, P < 0.001) and increased client satisfaction with care (mean difference 4.3, 95% CI 3.3–5.2, P < 0.001). Post-MBT emotional reactivity (mean difference −6.3, 95% CI −8.4 to −4.3, P < 0.001), psychiatric symptom distress (mean difference −5.2, 95% CI −6.8 to −3.7, P < 0.001) and impaired social functioning (mean difference −0.7, 95% CI −1.2 to −0.3, P = 0.002) were significantly lower than pre-treatment. Improved mentalising capacity predicted improvements in emotional reactivity (β = −0.56, P < 0.001) and social functioning (β = −0.35, P < 0.001). Conclusions Short-term MBT as a low-intensity treatment for personality disorder was associated with positive pre–post treatment changes in social and psychological functioning. MBT as deployed in this out-patient service expands access to personality disorder treatment.