Consistent with the need for implementation research and prevention programming for students in schools, the current study evaluated the implementation and outcomes of Strong Start, a social-emotional learning program, as a supplemental intervention for students in kindergarten through second grade at risk for developing emotional and behavioral problems. This intervention took place during the first year of a county-wide restructuring of mental health supports and was part of a multi-tiered system of supports provided in schools. A mixed method program evaluation was conducted to examine four areas of interest. First, the contextual factors related to program adoption were examined; second, program implementation was evaluated; third, student outcomes were assessed; and finally, the social validity of the Strong Start curriculum was evaluated. Results indicate that some contextual factors were related to decisions to adopt and implement Strong Start, that implementation integrity varied but was adequate overall, and that the curriculum was viewed positively by multiple stakeholders. However, no significant differences were detected between treatment and comparison groups in this first year of program implementation. These results are discussed in reference to theoretical implementation models and used to elucidate the process and challenges encountered in the first year implementation of large-scale initiatives across multiple schools. Limitations of this study and directions for future research are discussed.