Substance use during the perinatal period and while parenting can pose a significant risk to children's safety and well-being. Mothers who have experienced child maltreatment are more likely to use substances than mothers without a history of maltreatment. This study explores how child welfare social workers experience supporting young, maltreated mothers struggling with substance use to prevent the intergenerational transmission of child maltreatment. Semi-structured in-depth interviews were conducted with four social workers working with young mothers with a history of maltreatment and substance use. Interpretative Phenomenological Analysis revealed two themes: (1) grappling with system challenges and (2) supporting strategies for disrupting intergenerational transmission of child maltreatment. The results highlight the need for systemic changes around support for social workers who work with young mothers who use substances and have a history of maltreatment, and substance use treatment and mental health programs themselves. Mothers need access to prenatal programs that are trauma-informed, non-judgemental and that support participants' basic needs and parenting skills.