Students' overall performance in country wide national examinations in Kenya has recorded below mean achievement as attested by Kenya Certificate of Secondary Education (KCSE) results. The number of students scoring grade D+ and below has consistently remained high compared with those attaining grade C+ and above which is the country's minimum university entry. Concerns have been raised regarding this trend in students' achievement. This study examined the performance of the principals' role in promoting teachers' professional development and learners' performance in Kirinyaga and Murang'a counties, Kenya. The study involved a sample of 205 principals and 367 teachers selected from secondary schools in the two counties. Researchers used validated questionnaires and interview schedules to collect data from teachers and principals and analysed data using descriptive and inferential statistics. The hypothesis was tested at p>0.5 level of significance using Pearson Product Moment Correlation. The study established that there was no statistically significant relationship between principals' role in promoting teachers' professional development and learners' performance. The study recommended that principals should ensure that teachers put into practice knowledge and skills learnt during in-service courses so that students benefit from the investment. The Ministry of Education should also take keen interest in how in-service courses are conducted.