Classroom reading instruction is the foundation of Multi-Tiered Systems of Support in Reading (MTSS-R) and the focus of many state reading reforms, initiatives, and policies. The purpose of this study was to evaluate whether comprehensive and coordinated Tier 1 core reading instruction was associated with higher early reading outcomes of students in grade K-3 in four elementary schools across four different school districts that were selected to participate in a state MTSS-R initiative. We used a quasi-experimental successive cohort design that compared student early reading outcomes after one and two years of implementation of comprehensive and coordinated Tier 1 classroom reading instruction to student outcomes during a baseline year during which schools implemented business-as-usual classroom reading instruction. We found that students experienced statistically significantly higher early reading outcomes on a composite measure of early reading outcomes after one year of implementation and larger effects after the second year of implementation.