The purpose of this study was to investigate the influence of school language policy on pupils‟ achievement in English composition in public primary schools in Trans-Nzoia West sub-county. The study adopted the descriptive survey research design. The target population was 1,715 class 8 pupils, 120 teachers of English and 9l head teachers. Purposive sampling techniques were employed to select 73 head teachers and 92 teachers of English who were involved in the study. The 313 pupils who participated in the study were selected using proportionate stratified sampling procedures. Data were collected using the teachers‟ questionnaires, the head teachers‟ interview schedule and pupils‟ English composition achievement test. The content and face validity of the three instruments were examined by the researcher in consultation with experts from the department of Curriculum and Education Management of Laikipia University. The English teachers‟ questionnaire and the English composition achievement test were piloted in 5 schools in Kwanza Subcounty and their reliabilities estimated using the Cronbach coefficient alpha method and the Kuder-Richardson 21 formula respectively. The reliability coefficients of English teachers‟ questionnaire and the English composition achievement test were .898 and .916 respectively. Data were analyzed with the aid of the Statistical Package for Social Sciences (SPSS) version 22. The hypothesis was tested at the .05 level of significance using simple regression. The results of the study revealed that school language policy influences pupils‟ achievement in English composition. The results of the study should assist teachers to manage and improve the quality of instruction in English composition. The results of the study should also help the policymakers in the Ministry of Education Science and Technology to develop a suitable language policy for implementation in all schools in Kenya.