This study sought to find out the influence of teachers-counsellors’ professionals development on guidance and counselling programme effectiveness in public secondary schools in Nakuru County, Kenya. The study employed descriptive and correlational research designs. Data was collected using teacher counsellors’ professional development questionnaire. The findings of this study revealed teacher counsellors have relatively high levels of professional development in guidance and counselling, and teacher counsellors’ professional development in guidance and counselling had a positive statistically significant prediction on school guidance and counselling programme efficacy. It was concluded that the guidance and counselling personnel who are actually full-time teachers with the extra role of counsellor have high levels of professional development in guidance and counselling, and the high levels of professional development positively influenced school guidance and counselling programme efficacy. Teacher counsellors in Kenya are recommended to enhance and manifest professional development in guidance and counselling in an effort to ensure effective school guidance and counselling programme.