Research into the relationship between English proficiency and academic achievement was built upon the concern of the academic success of international students studying in English-speaking countries, raising the question if such a relationship exists in domestic EFL learners studying at a university in their home country. This study, therefore, expands the exploration to the investigation among EFL learners in a non-English speaking country, i.e., Thailand by considering the variables at a micro-level across general English courses involving course grade and academic achievement--grade point average (GPA). The analysis involved the data of 2,150 students studying six different English courses from various academic majors. These results revealed significant correlations and strong predictive powers of students' grades in English courses on their GPAs across a year of study and proficiency. The findings sustain the argument that although various factors affect students' academic achievement, English proficiency seems to have predictive power. Nowadays English-medium instruction (EMI) has attracted a myriad of interests from universities in non-English-speaking countries. Therefore, the findings of this study suggest that improving students' English proficiency would lead them to obtaining better academic achievement.