In 2009 the Philippines Department of Education issued Order No. 74, “Institutionalizing Mother Tongue-Based Multilingual Education (MTB-MLE),” calling for the use of the learners’ mother tongues (MTs) in the early primary grades for improving learning outcomes. In 2012, the MTB-MLE policy was rolled out nationally in all Grade 1 (G1) classrooms. By the 2014– 2015 school year, all public schools were expected to be using one of 19 mother tongues as the medium of teaching and learning (MoTL) from Kindergarten (KG) through G3. The objective of this study was to provide insight into the relationships between the teachers’ and students’ language usage, the MTB-MLE policy implementation, and student reading outcomes, especially in areas with linguistically heterogeneous populations. It sought to examine how language usage in the classroom conforms to or diverges from the MTB-MLE policy after six years of implementation, which factors are associated with higher policy implementation, and how language usage by teachers and students relates to student learning outcomes.