Master Teachers, positioned at the Highly Proficient level of the Philippine Professional Standards for Teachers (PPST), play a vital role in strengthening instructional quality and supporting teacher growth. This study examined the influence of Master Teachers’ instructional leadership skills on teacher development in Maitum 1 and Kiamba 1 Districts, Sarangani Province, using a convergent parallel mixed-methods design. Quantitative data analyzed through descriptive statistics and Spearman’s rho revealed that instructional leadership skills were highly demonstrated (M = 3.26) and teacher development was strongly impacted (M = 3.42), with a highly significant positive relationship between the variables (r = 0.798, p < .001). Qualitative findings complemented these results by describing Master Teachers as mentors, instructional guides, curriculum facilitators, and role models who foster teacher growth through feedback, coaching, collaboration, and reflection. The integration of findings showed strong convergence, confirming that effective instructional leadership enhances teachers’ competence, confidence, motivation, and professional engagement, while highlighting the need to further strengthen differentiated instruction, technology integration, instructional innovation, and structured support for newly hired teachers.