The stressful nature of parenting infants exacerbates the characteristics of Borderline Personality Disorder (BPD). Consequently, mothers with BPD tend to be emotionally dysregulated, respond impulsively to their infants, and have poorer mother–infant relationships. Few parenting interventions target the specific skill deficits observed in mothers with BPD. This study explored the differences in parental reflective functioning (PRF) and mother–infant relationship quality at baseline and following a 24‐week, group parenting intervention for mothers with BPD. PRF and mother‐infant relationship quality were assessed from quantitative (N = 23) and qualitative (N = 32) perspectives. Quantitative data (Parental Reflective Functioning Questionnaire) showed a significant improvement in one of the three subscales, Interest and Curiosity, between baseline and post‐intervention, and a significant moderate positive association between the subscale Certainty of Mental States and maternal‐infant interaction quality post‐intervention. Improvements in mother‐infant relationship quality were not evident from the observational measure, Nursing Child Assessment Satellite Teaching scale. In contrast, semi‐structured interview qualitative data found maternal improvements in parental reflection, coping strategies implemented post‐intervention, and quality of mother–infant relationships. Overwhelmingly positive intervention feedback suggested perceived maternal benefits of group format and skills taught. Future studies with larger sample sizes would allow further clarification of such parenting interventions for mothers with BPD.