The aim of this study was to investigate the influence of mother tongue on students’ academic performance in English in public secondary schools in Turbo Sub- County Kenya. The study was based on the following research questions: How does the first language interfere with students’ academic performance in English in Turbo Sub-County?What is the perception of teachers and students on the influence of mother tongue on students’ academic performance in English in Turbo Sub-County?What is the influence of school policies on the speaking of mother tongue on students’ academic performance in English in Turbo Sub-County? The study was done in the months of January 2024 targeting mainly the principals, teachers and students in the schools involved. The study was interested in both qualitative and quantitative data and as a result, a descriptive survey research design was used. From the study findings, the following conclusions were reached by the researcher; Mother tongue interference reduces student confidence resulting into lower class performance; mother tongue interference contributes greatly to poorly constructed responses to questions. There are backgrounds that negatively influence the class outcomes of students in English. Decisive policies ensure that the speaking of mother tongue is reduced significantly in secondary schools. It was recommended that schools should introduce programs that discourage the use of mother tongue as a way of reducing its negative effects. Students should be informed at all times of the dangers that their first languages and their own perceptions about language which may have on the learning of English as a second language so that they are able to learn properly. Further research should be done on the influence of mother tongue on the effectiveness of students in secondary schools.