This study aimed to investigate the effects of English medium of instruction on the educational outcomes of students in Nigerian higher education institutions. Utilising a mixed-methods research design, the study involved a sample of 1,200 students and 100 lecturers from three universities, with data collected through structured surveys and in-depth interviews. The analysis revealed three major findings: (1) Higher English proficiency positively correlated with better academic performance; (2) Students and lecturers had mixed attitudes towards English-medium instruction, with students often struggling with language barriers; (3) Best practices in language support, such as interactive workshops and language labs, were identified, though their effectiveness varied. The study concluded that while English-medium instruction generally supports academic success, it also presents challenges related to language proficiency and cultural relevance. The implications suggest that educational institutions should enhance language support programmes, integrate local languages and cultural contexts into the curriculum, and ensure effective implementation of language instruction policies. Addressing these aspects can improve student outcomes and create a more inclusive educational environment.