The primary goal of this study is to evaluate the efficacy of professional development (PD) and implementation fidelity on the performance of middle school students in mathematics within an online learning environment. Using a quasi-experimental design, the research compares educational outcomes between control and experimental groups post-intervention. The experimental group participated in a two-quarter web-based treatment using the Novel Online Learning Environment (NOLE), while the control group continued with traditional teaching methods. The most important contribution of this study is demonstrating that targeted professional development and high implementation fidelity significantly enhance middle school students' mathematics performance in an online learning environment. The results show that the experimental group outperformed the control group immediately following the intervention and maintained this advantage over a two-year period. These findings suggest that integrating PD and high implementation fidelity contributes to lasting educational benefits in mathematics learning. Implications for further research include investigating the long-term effects of PD beyond the immediate post-intervention phases and exploring the impact of similar interventions across different subjects and educational settings. The study also underscores the importance of continuous support and resource allocation to maintain implementation fidelity, highlighting its critical role in successfully adopting innovative educational technologies and methodologies. This research enhances the understanding of strategic PD and its significant role in improving the effectiveness of online mathematics education.