This paper examines the impact of academic inbreeding, on language students‟ exposure to global English vocabularies and intelligibility in Nigeria‟s tertiary institutions. Academic inbreeding is the practice of retaining the products of an institution to teach as lecturers in the same institution after graduation. It is hypothesized that this phenomenon may limit language learners‟ exposure to diverse linguistic perspectives despite any language teaching approach employed by lecturers. The survey method was employed in gathering data from both students and lecturers. Five institutions in South West Nigeria (OAU, Ile Ife, AAU, Akungba, OOU, Ago Iwoye, AFUED, Ondo, and WUSTO, Ondo) were used as case study. The relationship between academic inbreeding and linguistic competence is explored in the study by collecting data through questionnaires to examine the impact of academic inbreeding on students‟ exposure to global English vocabularies and intelligibility. The findings align with the position of this paper that academic inbreeding negatively affects language students‟ vocabulary development. Based on the findings, it is recommended that governments provide sufficient financial support to universities to enable lecturers to go on sabbatical leave and teach in other institutions, facilitate lecturers‟ participation in international exchange programmes as visiting teachers, and equip language laboratories with modern gadgets to facilitate students‟ access to online lectures from foreign institutions