Insecure attachment has been described as mediating the relationship between childhood trauma and dysfunctional personality traits in different mental disorders. Despite the role insecure attachment and childhood trauma have independently demonstrated to play as determinants of borderline personality disorder, less is known about the mediating mechanisms explaining these associations. For the first time, we assessed adult attachment, childhood trauma and dimensional personality pathology in a sample of outpatients with borderline personality disorder and tested whether the association between childhood trauma and personality dysfunction was at least partially attributable to insecure attachment. The results showed that attachment anxiety fully mediated the relationship between specific types of trauma (emotional abuse and physical neglect) and emotional dysregulation. Further, emotional abuse was both directly associated with dissocial behaviour and indirectly via attachment anxiety (partial mediation). Emotional abuse has been described as an essential environmental factor for the development of borderline personality disorder and emotional dysregulation, on its part, as the core feature of the condition. Our results indicate that attachment anxiety explains the link between these central aspects of borderline personality disorder. Our findings are consistent with previous research and current etiological understanding of the condition and provide support for recommending a careful assessment of childhood traumatic experiences and adult attachment style to gain a more comprehensive insight into the symptoms and its heterogeneity. As a secondary aim, we assessed the effect parental mental illness may have in these mediation models, but no significant influence on childhood trauma, attachment or personality was found.