This dissertation explores the predictors of academic success among undergraduate students in English-Medium Instruction (EMI) higher education in Kazakhstan. Despite the rapid adoption of English as a medium of instruction, comprehensive research on its impact remains scarce. This study aims to fill this gap by examining whether secondary education in English provides an advantage for academic achievement in EMI university programs. Utilizing a mixed-methods approach, quantitative data were collected through a survey of 97 undergraduate students, and qualitative data were gathered via semi-structured interviews with 7 EMI instructors. The quantitative analysis, conducted using Ordinary Least Squares (OLS) regression, identifies several predictors of academic outcomes, including English proficiency, financial aid status, and year of study. Notably, English proficiency emerged as a significant predictor of higher GPA, while financial aid recipients were less likely to have retakes. The language of instruction at the secondary school level did not significantly predict academic success in EMI settings. Qualitative findings corroborate the quantitative results, emphasizing the importance of English proficiency and highlighting the role of secondary school type. Instructors noted that students from specialized schools for gifted children tend to perform better in EMI programs. Additionally, factors such as parental support, student motivation, and the level of secondary school education were considered influential. This research contributes to the literature on EMI by providing evidence from the Kazakhstani context and offers practical implications for policy and practice, suggesting targeted language preparation programs for students who did not attend EMI secondary schools. Future research should consider broader and more diverse samples to enhance the generalizability of the findings.