The number of K-12 classrooms adopting blended learning models is rapidly increasing and represents a cultural shift in teaching and learning; however, fidelity of implementation of these new blended learning programs varies widely. This study aimed to examine the role of teacher engagement in student motivation and achievement in a blended learning environment. Reading skill data were analyzed from 19,366 students across 624 schools led by teachers defined as engaged users of a blended learning reading program (Lexia Reading Core5 [Core5]). Results showed significant improvements in reading skills during the analyzed period for the students of the engaged teachers in comparison to neighboring classrooms (171,850 students in the same 624 schools) of less engaged teachers.