Exposure to traumatic experiences among youth is a serious public health concern. A trauma-informed public behavioral health system that emphasizes core principles such as understanding trauma, promoting safety, supporting consumer autonomy, sharing power, and ensuring cultural competence, is needed to support traumatized youth and the providers who work with them. This article describes a case study of the creation and evaluation of a trauma-informed publicly funded behavioral health system for children and adolescents in the City of Philadelphia (the Philadelphia Alliance for Child Trauma Services; PACTS) using the Exploration, Preparation, Implementation, and Sustainment (EPIS) as a guiding framework. We describe our evaluation of this effort with an emphasis on implementation determinants and outcomes. Implementation determinants include inner context factors, specifically therapist knowledge and attitudes (N = 114) towards evidence-based practices. Implementation outcomes include information on rate of PTSD diagnoses in agencies over time, number of youth receiving TF-CBT over time, and penetration (i.e., number of youth receiving TF-CBT divided by the number of youth screening positive on trauma screening). We describe lessons learned from our experiences building a trauma-informed public behavioral health system in the hopes that this case study can guide other similar efforts.