This study examined teachers’ perceptions of principals’ instructional leadership behaviors and their influence on teacher role performance in public secondary schools in Nakuru and Narok Counties, Kenya. The study sought to determine how instructional leadership practices such as supervision, promotion of professional development, management of instructional programs, and creation of positive school culture influence teachers’ classroom teaching, student mentoring, and assessment practices. The study was anchored on Weber’s (1996) Instructional Leadership Model and adopted a convergent parallel mixed-method design. The target population consisted of principals and teachers drawn from public secondary schools in the two counties. Data were collected using questionnaires and interview schedules and analyzed through descriptive statistics and thematic analysis. The findings revealed that teachers generally perceived principals’ instructional leadership behaviors positively and that these behaviors had a moderate influence on teacher roleperformance. Specifically, management of instructional programs, promotion of professional development, and supportive school culture contributed significantly to teacher effectiveness. The study concludes that strengthening principals’ instructional leadership practices is essential for improving teacher role performance and overall school effectiveness. The study recommends continuous leadership training and capacity-building programmes for principals and teachers in Kenyan secondary schools.