English-medium universities often accept scores from various English language proficiency (ELP) tests as evidence of ELP from non-English background students. This practice raises the question of how these tests compare in terms of their ability to predict academic achievement. This longitudinal study addresses this question by examining the strength of the associations between total scores on the IELTS Academic and the TOEFL iBT, on one hand, and the academic achievement of 6481 non-English background undergraduate students in the first 10 semesters of their study at a Canadian, English-medium university, on the other. Findings revealed that the association between ELP and academic achievement varied across ELP tests and disciplines. Furthermore, students with different IELTS scores exhibited significantly different grade point average (GPA) trajectories over time. Specifically, students with lower IELTS scores tended to exhibit a more substantial decline in GPA over the 10 semesters compared to students with higher scores who displayed less decline in GPA, suggesting greater resilience. The findings and their implications for research concerning the relationship between ELP and academic achievement over time are discussed.