This study investigates the prevalence of maternal substance use disorder (SUD) within the Pennsylvania child welfare system and the impact of formal child welfare interventions on mothers’ access to SUD treatment. Using linked Medicaid claims and child welfare records for 46,484 mothers on Medicaid between 2015 and 2018, the study applies a difference-in-differences framework to evaluate how in-home services, foster care services, and no formal services affect SUD treatment uptake. Findings reveal that 62% of mothers involved in the child welfare system had any SUD, primarily related to opioid and polysubstance use. Mothers who received formal child welfare interventions were significantly more likely to utilize outpatient and inpatient SUD treatment in the 12 months following system contact, compared to those without formal intervention. The results highlight the role of child welfare services in facilitating caregiver access to SUD treatment but underscore the need for alternative approaches to engage and retain parents in treatment as states consider reducing formal child welfare system involvement in addressing parental substance use.