Background: The implementation of evidence-based practices (EBPs) in community settings appears to result in reduced benefit relative to controlled trials. This difference in outcomes may be attributable in part to engagement challenges therapists encounter when delivering EBPs to low-income ethnic minority youth and families. Objective: The current study sought to identify therapist, client, and session characteristics associated with therapist-reported engagement challenges in therapy sessions, as well the associations between two types of client engagement challenges and therapists' self-reported ability to deliver the EBP in sessions within a system-driven implementation in public children's mental health services. Method: One hundred and three therapists reported on two types of engagement challenges--Limited Client Engagement and Expressed Client Concerns--in 702 sessions with 274 clients. Results: Results indicated that therapists reported a higher frequency of Limited Client Engagement in sessions with male clients and in sessions where the youth was present, and by therapists with smaller caseloads. No variables significantly predicted Expressed Client Concerns. Both types of engagement challenges were negatively associated with therapists' report of their ability to carry out intended activities in the same session. Conclusions: Findings suggest that therapists may benefit from learning strategies to address these two distinct types of engagement challenges encountered in implementation of EBPs with diverse families in community settings.