Abstract Early life stress is said to play a critical role in the development of borderline personality disorder (BPD) and major depressive disorder (MDD), but the underlying mediating factors remain uncertain. This study aimed to investigate self-reported childhood trauma, emotion regulation difficulties and their associations in a sample of BPD (n = 49) and MDD (n = 48) patients and healthy control participants (n = 63). Multiple regressions were used to evaluate the impact of the quality and severity of self-reported childhood trauma on self-reported emotion regulation. The results support an association between self-reported maltreatment experiences, especially emotional abuse and neglect, and emotion regulation difficulties. Additional analyses show that emotion regulation difficulties influence the association between self-reported emotional abuse and acute symptomatology in the BPD subgroup. Emotion regulation difficulties may be one pathway through which early life stress, particularly emotional abuse, increases the risk for developing BPD symptomatology.