Language acquisition and learning literatures favour mother-tongue education policies, particularly in the early years of schooling. However, English Medium Instruction (EMI) remains a popular language policy position in multilingual contexts. This paper studies EMI transition policies using a longitudinal dataset from Ethiopia, by leveraging regional variation in education language policy reforms for causal identification. Employing a dynamic value-added model, this paper shows that students in schools using English as a medium of instruction have lower mathematics test scores (−0.25 standard deviations) compared to students in mother tongue education schools. Furthermore, English medium learners do not perform any better in their English test scores compared to mother tongue learners. These findings are in line with the international education literature on skill development, learning and second language acquisition. The main results support the prolonged utilisation of mother tongue instruction in primary education. These results are particularly relevant for policy makers in linguistically diverse contexts.