The study examined the influence of school heads' clinical supervision practices on teachers' professional competency development in public secondary schools in Tanzania. It was a mixed-methods research approach that involved Tanzanian school heads and teachers. Survey data from 94 participants were quantitatively analysed, and narrative data from 28 informants were subject to content analysis. Based on narrative findings, clinical supervision was the factor in developing teachers' professional competencies: searching for teaching and learning resources, understanding the subject matter, organizing lessons, conducting interactive teaching and learning, and evaluating students' learning. The descriptive results showed pre-observation of the lesson plans (M = 1.79) ranked at strongly agreed responses. Pre-observation of schemes of work, classroom observation, supervisory feedback, teachers' professional support, and post-observation had a range of (M = 1.86) and (M = 2.06) ranked at agreed responses. These findings detail that school heads' clinical supervision practices positively influenced teachers' professional competency development. Therefore, the present study contributes to the existing literature of clinical supervision that each clinical supervision technical aspect has the potential to enhance teachers' teaching professional competencies development in the workplace. These findings exceptionally increase the need to strengthen the clinical supervision practices that centrally map teachers' professional needs and development.