Childhood maltreatment (CM) may have a long-term effect on emotion regulation. This study aimed to explore the relationship between CM and emotion dysregulation (ED) in a heterogeneous population. Four hundred seventy French-speaking outpatients (N = 279 ADHD, N = 70 BPD, N = 60 ADHD + BPD, N = 61 clinical controls) completed the Emotion Reactivity Scale (ERS), the Cognitive Emotional Regulation Questionnaire (CERQ), the Childhood Trauma Questionnaire (CTQ), and the Relationship Scales Questionnaire (RSQ). Reports of childhood maltreatment experiences were significantly associated with increased levels of emotion reactivity in all our groups and in the whole population, with a greater use of non-adaptive cognitive emotion regulation strategies and insecure attachment patterns. Emotional abuse showed the strongest effect. Further analysis indicated that an anxious attachment style significantly mediated the relationship between CM and the use of non-adaptive cognitive emotion regulation strategies and emotion reactivity. The results of our study suggest an impact of CM on ED and a potentially marked effect of emotional abuse. They also indicate a potentially mediating role of insecure attachment in the relationship between a history of childhood abuse and emotion reactivity and a higher use of non-adaptive cognitive emotion regulation strategies in adulthood.