Previous research has demonstrated the importance of ensuring that programs are implemented as intended by program developers in order to achieve desired program effects. The current study examined implementation fidelity of Pathways to Health (Pathways), a newly developed obesity prevention program for fourth- through sixth-grade children. We explored the associations between self-reported and observed implementation fidelity scores and whether implementation fidelity differed across the first 2 years of program implementation. Additionally, we examined whether implementation fidelity affected program outcomes and whether teacher beliefs were associated with implementation fidelity. The program was better received, and implementation fidelity had more effects on program outcomes in fifth grade than in fourth grade. Findings suggest that implementation in school-based obesity programs may affect junk food intake and intentions to eat healthfully and exercise. School support was associated with implementation fidelity, suggesting that prevention programs may benefit from including a component that boosts school-wide support.