Partners to Lead (PtL) is a school leadership professional development (PD) project funded by a 5-year Education Innovation and Research grant and implemented by the DuPage, Illinois, Regional Office of Education (ROE) in 37 public elementary, middle, and high schools in four Illinois ROEs. The American Institutes for Research® (AIR®), the independent evaluator of PtL, has completed an implementation and impact study of the program. This final report summarizes the PtL program and AIR's evaluation methods; the extent to which PtL was implemented as designed; and findings on PtL's impact on student achievement, changes to principal effectiveness as measured by the Illinois 5Essentials Survey, and principal retention. All 37 schools recruited to participate in PtL remained in the program across all 3 years of the intervention, and principals generally expressed satisfaction with the components of PtL. However, analyses indicate that PtL did not have a statistically significant positive impact on principal leadership effectiveness, student achievement, or principal retention. From March 2020 through the end of the intervention, some PtL principals may have become somewhat disengaged from the full PtL PD and coaching due to competing professional and personal priorities resulting from the COVID-19 pandemic, which may have attenuated the impact of PtL. Nonetheless, students in intervention schools scored 0.104 standard deviation higher in math in spring of the final year of the intervention, and the p value for the estimated positive impact on student math achievement was 0.06.