The roles of school principals have been focal in ensuring and promoting the performance of students in their schools, achieving and sustaining quality education. This has been influenced by the paradigm shifts and global trends to ensure that the quality agenda as espoused in various international and national policies such as Sustainable Educational Goals. A cardinal role is ensuring on teacher supervision in curriculum implementation in their schools. The design of principals’ transformational leadership practices is evidenced to contribute in increased learners’ performance in national examinations as well as fostering transformative school environment. Instructional Leadership is significant in fostering teachers’ instructional practices and subsequently students’ learning and achievement. The aim of this study was to examine the role of principals in the supervision of teaching and its influence on promoting learners’ performance. The study adopted the descriptive survey research design with a target population of 436 principals and 8,049 teachers from secondary schools in Murang’a and Kirinyaga Counties of Kenya. Purposive sampling was used to select 205 principals while 367 teachers were selected using stratified and then simple random sampling methods. The researcher used open and closed-ended questionnaires to collect quantitative data from the teachers and semi-structured interview schedules to gather qualitative data from principals. A pilot study of the instruments was conducted in two schools in Kiambu County of Kenya and Cronbach’s alpha coefficient with a threshold of 0.7 was used to determine the internal consistency of the items. The instruments yielded a reliability coefficient of 0.962, hence were considered reliable. Quantitative data was analyzed using descriptive and inferential statistics with the aid of Statistical Package for Social Sciences (SPSS) version 20. Hypotheses were tested at p>0.5 level of significance using Pearson Product Moment Correlation and t-test determined whether or not the means were statistically significant. Multiple regression analysis was used to determine whether the principals’ instructional leadership practice is a predictor of learners’ performance. The findings of the study were that the level of supervision of teaching was moderate and therefore had significant impact on learners’ performance. The study recommended that there is need for supervision of teaching. The research findings are of significance to principals in designing instructional strategies to improve learners’ performance and to policy makers in the education sector in designing policies that can support effective instructional leadership practices in schools.