Mothers with substance use issues often struggle with other interrelated issues, including interpersonal violence and child maltreatment, poverty, and trauma. The Circle of Security Parenting intervention (COS-P) is designed to enhance child secure attachment to their parent/caregiver, though evaluation of the intervention has found somewhat mixed findings, particularly for vulnerable families living in conditions of risk (e.g., substance use, interpersonal violence). The aim of the current study was to examine the experiences of mothers who attended the COS-P intervention, as implemented within a comprehensive community-based program for mothers with substance use issues and their young children in Toronto, Canada. Fifteen mothers attended the COS-P intervention (93% completion rate) and 6 participated in focus groups to better understand their experiences in the intervention and subsequent changes. A focus group was also conducted with program staff who delivered the intervention. Phenomenological analyses indicated changes to mothers' internal processes for parenting and the mother-child relationship, changes to external behaviors regarding parenting and relationships, changes in children's behavior, and mothers' ability to apply COS-P concepts outside of the program delivery. Findings from staff echoed mothers' experiences, with staff also indicating the ways in which the COS-P intervention could be effectively implemented within a wraparound support program that provides additional services to mothers with substance use issues and their young children. By centering the lived experiences of mothers and their young children, findings have implications for enhancing mother-child relationships among families experiencing substance use and interpersonal violence.