The limited implementation of a trauma-informed approach in Canadian mental health care may partly reflect limited knowledge regarding the prevalence and/or impact of childhood trauma among service users. This study examined the prevalence of childhood maltreatment in a sample of 136 Canadian adult mental health service users, and examined its cumulative burden in relation to psychiatric distress and suicidality. Eighty-six percent reported some form of childhood maltreatment, with the majority experiencing multiple traumas. Cumulative trauma was significantly associated with psychiatric distress and history of suicide attempt. The findings suggest implications for Canadian mental health practice, policy, and further research.