This article reports a quantitative empirical study that investigated whether English language proficiency increases over time when studying academic content through English Medium Instruction (EMI). It was also investigated whether an increase in proficiency predicts EMI academic achievement. Student English language test score data and Grade Point Average (GPA) data were collected from a public university in Turkey. Two academic subjects were compared: Business Administration (a Social Science subject, n = 81) and Mechatronics Engineering (a Mathematics, Physical and Life Sciences subject, n = 84). Results showed that in both subjects, English language proficiency statistically significantly improved over a four-year period of studying through English. Furthermore, this improvement predicted EMI academic achievement; meaning that the more proficient students became in English, the higher they achieved in their EMI academic studies. This provides evidence for policymakers, EMI practitioners, and language professionals around the world that English does improve when studying academic content through English, and that this improvement has a positive effect on content learning outcomes. Implications of these findings, and suggestions for further research are discussed.