Group-based interventions are effective and widely used with parents of children with autism spectrum disorder (ASD). Family treatment research demonstrates that parental treatment engagement is a critical factor in determining intervention outcomes. At the present time, practitioners and researchers lack coherent conceptual frameworks of treatment engagement in group-based interventions. The purpose of this investigation was to explore the construct of treatment engagement and those factors that contribute to engagement in a group-based intervention for parents of children with ASD. Semi-structured telephone interviews were conducted with parents who participated in a group-based intervention for parents of children with ASD. Interview questions elicited parents’ experiences throughout the intervention and the factors that either detracted from or promoted their treatment engagement. Qualitative analysis yielded seven thematic categories that are organized into a conceptual framework of treatment engagement in group interventions. Specifically, we suggest that intervention characteristics (practitioner behaviors and program logistics), relational factors (group dynamics and therapeutic alliance), and external barriers/daily stresses influence parents’ attitudinal and behavioral treatment engagement. These findings can assist practitioners who are designing and implementing group interventions to more effectively engage parents of children with ASD in treatment. The resulting framework also has important implications for the conceptualization and application of treatment engagement more broadly. Future directions for research are discussed.