Sheng, a hybrid language combining English, Swahili, and local dialects, is widely used among urban youths, particularly school-going students. This language serves as a means of identification, secrecy, and socialization, influencing the students' English grammar usage. While numerous studies have explored the effect of Sheng on native and official languages as well as societal perceptions, there has been limited research on its influence on the learning of English grammar in public secondary schools. Therefore, this paper focuses on the contributing factors to the widespread use of Sheng and subsequent poor performance of English grammar, in public secondary schools at Roysambu Sub-County in Nairobi County, Kenya. Universal Grammar Theory of Second Language Learning by Chomsky (1985) is used as an analytical tool. The research aims to determine how the popularity of Sheng influences students' grammatical usage in English, examine its effect on grammar performance, and evaluate its contribution to poor grammatical competence among learners in public secondary schools. The findings reveal that school location and student’s social background lead to the spread of sheng in the learning institution. This leads to a negative attitude towards English grammar hence poor performance. The findings hold significance for the Ministry of Education and public secondary principals in identifying causes of the widespread use of Sheng in learning institutions, leading to poor performance in English grammar. Additionally, the findings provide a foundation for future studies and inform curriculum developers in devising strategies to address the prevalent usage of Sheng in learning institutions.