Objective: Community therapists inevitably adapt evidence-based practices (EBPs) to meet the needs of their clients and practice settings. Yet, the implications of spontaneous, therapist-driven adaptations for EBP implementation outcomes are not well understood. We used a sequential QUAN → qual mixed-methods design to examine how different types of therapist-described adaptations were associated with observerrated extensiveness of therapist delivery of EBP content and technique strategies at the session level. Method: Data were drawn from an observational study of a system-driven implementation of multiple EBPs into public children's mental health services. Community therapists (n = 103) described adaptations they made in 680 sessions with 273 clients (50.92% female, 49.08% male, M age = 9.72 years, 70.70% Hispanic/ Latinx). Coders classified therapist-described adaptations into five types: (a) Modifying Presentation, (b) Integrating, (c) Extending, (d) Reducing, and (e) Generalizing. Independent observers rated the extensiveness of EBP strategy delivery from session recordings using the EBP Concordant Care Assessment (ECCA) Observational Coding System. Results: Quantitative analyses using multilevel regression revealed that Modifying Presentation adaptations were associated with higher extensiveness of EBP technique delivery, whereas Extending adaptations were associated with lower extensiveness of EBP content and technique delivery. Qualitative analysis of adaptation descriptions identified explanations for the quantitative findings. Conclusions: Findings suggest that Modifying Presentation adaptations, associated with higher extensiveness, involved creative use of activities and materials, language modification, and personalization of EBP content to meet clients' diverse needs, whereas Extending adaptations, associated with lower extensiveness, involved slowing EBP pacing in response to client challenges. Implications for provider training are discussed.What is the public health significance of this article? Community therapists adapt evidence-based practices (EBPs) when implemented in public children's mental health services, yet how such adaptations impact EBP delivery is unclear. Findings from the present study suggest that sessions in which community therapists tailored presentation of EBP concepts in creative ways to meet clients' diverse needs were associated with higher extensiveness of EBP technique delivery. In contrast, sessions in which therapists slowed down intervention pacing and relied on repetition of didactic content were associated with lower extensiveness of EBP content and technique delivery. These findings may have implications for the targeted design of supportive training strategies in the community implementation of EBPs for diverse youth.