The present study extends the body of evidence regarding the effectiveness of day hospital Mentalization-Based Treatment (MBT) by documenting the treatment outcome of a highly inclusive group of severe borderline personality disorder (BPD) patients, benchmarked by a carefully matched group who received other specialized psychotherapeutic treatments (OPT). Structured diagnostic interviews were conducted to assess diagnostic status at baseline. Baseline, 18-month treatment outcome and 36-month treatment outcome (after the maintenance phase) on psychiatric symptoms (Brief Symptom Inventory) and personality functioning (118-item Severity Indices of Personality Problems) were available for 29 BPD patients assigned to MBT, and an initial set of 175 BPD patients assigned to OPT. Propensity scores were used to determine the best matches for the MBT patients within the larger OPT group, yielding 29 MBT and 29 OPT patients for direct comparison. Treatment outcome was analysed using multilevel modelling. Pre to post effect sizes were consistently (very) large for MBT, with a Cohen's d of −1.06 and −1.42 for 18 and 36 months, respectively, for the reduction in psychiatric symptoms, and ds ranging from 0.81 to 2.08 for improvement in domains of personality functioning. OPT also yielded improvement across domains but generally of moderate magnitude. In conclusion, the present matched control study, executed by an independent research institute outside the UK, demonstrated the effectiveness of day hospital MBT in a highly inclusive and severe group of BPD patients, beyond the benchmark provided by a mix of specialized psychotherapy programmes. Interpretation of the (large) between condition effects warrants cautionary caveats given the non-randomized design, as well as variation in treatment dosages.