The aim of this research was to establish the extent of code switching in the mastery of English language in secondary schools in Mbooni West sub county. The extent of code switching is based on language use by both the teachers and the students in the classroom setup. The study applied Communication Accommodation Theory developed by Giles and Coupland. The study adopted a descriptive Survey Research design. The area of study was 15 selected schools in Mbooni west Sub County. The methods of collecting data were: an observation schedule, teachers and students’ questionnaires. The target population for this study was the teachers and students in 41 public schools in Mbooni West Sub County. The sample size of the study was 300 students and 60 teachers of English from 15 schools that were sampled through purposive and simple random sampling methods. Both qualitative and quantitative methods were applied in the analysis of the data. The findings of the study revealed that code switching enhances the teaching/learning of English-speaking skills. The teachers use code switching for curriculum access, for classroom management discourse and for interpersonal relationships. Learners used code switching as a communicative strategy, for affective functions and for socio pragmatic functions.