Background: In 2012, Philadelphia's Department of Behavioral Health and Intellectual disAbility Services (DBHIDS) developed an initiative to implement an evidence-based treatment for posttraumatic stress disorder (PTSD), trauma-focused cognitive behavioral therapy (TF-CBT), across the city's behavioral health system. This report evaluates the initiative's 10-year implementation and effectiveness outcomes. Method: The Exploration, Preparation, Implementation, and Sustainment framework guided our implementation evaluation. The implementation outcomes include adoption, reach, and sustainment; these were obtained during regular evaluation data collection from publicly funded behavioral health agencies participating in the TF-CBT initiative. We analyze effectiveness outcomes (i.e., changes in PTSD symptoms) from a subset of patients receiving TF-CBT, which were collected in 6-month intervals by our research team between 2013 and 2021. Results: From 2012 to 2021, DBHIDS trained 478 clinicians in TF-CBT across 20 behavioral health agencies. During this time, 23,401 youths were screened for potentially traumatic events and PTSD symptoms, and 7,550 youths received TF-CBT. Through the TF-CBT initiative, the city expanded the network of TF-CBT providers from 3 to 20 agencies. DBHIDS sustained this network by maintaining the participation of 16 behavioral health agencies over the course of a decade. The subset of 202 youths who were evaluated to assess TF-CBT effectiveness was drawn from 94 therapists and 20 agencies across Philadelphia. All participating youths completed a baseline assessment, and 151 (75%) completed at least one follow-up assessment. Linear mixed-effects models accounting for observations nested within participants and nested within clinicians found that treatment significantly reduced PTSD symptoms. Conclusion: Between 2012 and 2021, DBHIDS successfully implemented and sustained TF-CBT across the city's behavioral health system. Adoption, reach, and sustainment of TF-CBT were high. Despite the considerable adverse experiences faced by youths seeking treatment in Philadelphia's behavioral health system, TF-CBT was effective. Future directions to improve TF-CBT implementation in the next iteration of the initiative are described.