Collaborative supervision helps teachers to develop professionally. Therefore this study which was conducted in 2017 sought to investigate the factors that affect Departmental Collaborative Supervision (DCS) and the Professional Development (PD) of Teachers of English language (ToEL) in Secondary Schools in Trans Nzoia County, Kenya. It was a follow-up study to the one done in Bungoma County in 2015-2016 in order to corroborate the findings. The study was based on the Mixed Methods Research Approach; it adopted the Constructivist, Cognitivist and Transformational theories of adult learning and the Pragmatic Philosophical Paradigm and the Explanatory Design. Proportionate stratified random sampling, simple random sampling and purposive sampling techniques were used to get the sample. The questionnaire and interview guide were used to collect data from teachers of English and heads of department from selected secondary schools in Trans Nzoia County. The sample involved 200 participants. The data were analysed using descriptive statistics. Findings revealed that most departments/schools experienced almost similar impediments towards DCS and PD of teachers of English. It can be concluded that DCS faces several impediments that negatively affect PD of ToEL. Consequently, it is recommended that teachers of English, departments/schools and the Ministry of Education, Science and Technology should adopt collaborative supervisory practices; and school and the government should provide needed support and resources in order to improve students’ learning and teachers’ professional development.