This study uses implementation fidelity data from PreK to 1st grade in the Boston Public Schools (BPS) to measure instructional alignment and examine whether stronger alignment is associated with sustained benefits of BPS PreK on children's language, literacy, and math skills through first grade. The study includes N = 498 students (mean age = 5.47, SD = 0.30 in K fall). Children who experienced strong instructional alignment across grades had faster gains in literacy (SD = 0.47) and math (SD = 0.28) skills through the spring of first grade compared with non-BPS PreK attenders. Mis-alignment predicted faster convergence in literacy skills. Results highlight that instructional alignment may help to sustain the initial benefits of PreK programs through first grade in a subset of outcome domains. Implications for further research measuring alignment in a broader range of settings and implications for practice are discussed.