In the recent past, external instructional supervision of teachers has been known to have an impact on the student’s performance and more especially on the KCSE performance. The purpose of this study is to investigate the influence of external instructional supervision on public secondary school teachers’ performance in the Ndhiwa sub-county. The researcher believes that the process of supervision is a collaboration among the stakeholders in Education. The researcher used simple random sampling to select 15 public secondary schools, 15 secondary school principals and 103 teachers; purposive sampling was used to automatically select instructional supervisors (one QASO and one TSC sub-county director) from the Ndhiwa sub-county. Questionnaires were used as a data collection instrument for teachers, principals, and supervisors (QASO) and an interview guide for the TSC sub-county director. The data collected was subjected to the statistical package for social sciences to generate descriptive statistics. Thematic analysis was also carried out on the qualitative data. The study established that teachers’ performance is improved if the supervisor is usually warm and friendly to teachers, always assists the teacher where they go wrong, and often tries to be fair, firm, open and transparent in their dealings with teachers. The study established that supervisors always possess the necessary problem-solving skills, skills to observe teachers, skills for designing instruments for evaluation and whether supervisors help schools promote peace among staff and students. The study established that supervisors visiting school could easily improve cooperation with teachers by giving guidance and advice; teachers regarded them as professional colleagues and not as subordinates or inferiors