Objective: Mothers with insecure attachment styles may have increased difficulty establishing secure attachment with their infant and may experience emotion regulation difficulties in the postpartum period. This study aimed to examine the mediating effect of emotion dysregulation (ED) on the relationship between maternal insecure attachment style and self-reported attachment to infant in women admitted to a mother-baby unit. Methods: At admission, women completed self-report questionnaires measuring attachment style, ED, postnatal attachment and postnatal depression (PND) symptoms. Descriptive statistics, correlations, and mediation analyses (PROCESS macro) were conducted. Results: Insecure maternal attachment style was found to predict postnatal attachment through ED. When PND symptoms were accounted for, this model was no longer significant, and instead a serial mediation found that ED predicted PND, which was in turn associated with reduced postnatal attachment quality. Conclusions: This study provides preliminary support for continued research into maternal ED as a predictor of adverse maternal and infant outcomes postnatally. Individuals with attachment insecurity experiencing emotion regulation difficulties may be at an increased risk of developing PND and in turn experiencing adverse mother-infant attachment outcomes. This has clinical implications for the screening, assessment and treatment of women experiencing mental health or attachment related concerns postnatally.