Advances in applied developmental science have contributed to the large literature on positive youth development (PYD) interventions. This study reports an investigation of a PYD program using an outcome-mediation evaluation model that drew on the treatment intervention science literature. The Changing Lives Program (CLP) is a community supported gender and ethnic inclusive PYD intervention framework. Using an empowerment approach, the CLP was implemented in a practice setting as a selective/indicated positive youth development program for multi-ethnic, multi-problem at risk youth in urban alternative high schools. The outcome-mediation evaluation model provided evidence in support of the direct outcome effects of the intervention. The model also proved effective in identifying at a micro process level a number of plausible causal mechanisms for use in the development of more conceptually complete models of the causal mechanisms that underlie intervention change. Implications for future development of theory informed empirically supported intervention models of PYD programs are discussed.