Patients with borderline personality disorder (BPD) have deficits in neurocognitive function that could affect their ability to engage in psychotherapy and may be ameliorated by improvements in symptom severity. In the current study, 18 patients with BPD completed neurocognitive tests prior to beginning mentalization-based therapy and again after 6 months of treatment. Twenty-eight nonpsychiatric controls were tested over the same period of time but received no intervention. Before starting treatment, patients performed lower than controls on tests assessing sustained attention and visuospatial working memory. After 6 months of treatment, patients showed significantly greater increases in sustained attention and perceptual reasoning than controls, with initial deficits in sustained attention among patients resolving after treatment. Improved emotion regulation over the follow-up period was associated with increased auditory-verbal working memory capacity, whereas interpersonal functioning improved in parallel with perceptual reasoning. These findings suggest that changes in neurocognitive functioning may track improvements in clinical symptoms in mentalization-based treatment for BPD.