The study investigated the relationship between headteachers' leadership styles and teachers' content mastery in public secondary schools in Lwengo District, Uganda. The research employed a correlational survey design with a quantitative approach, targeting a population of 420 teachers and 15 headteachers from 15 public secondary schools. Data were collected using structured questionnaires validated for reliability and analyzed using descriptive statistics, Pearson correlation, and regression analysis. The findings revealed that transformational leadership style had a strong positive correlation with teachers' content mastery (r=0.768, p<0.01), democratic leadership showed a moderate positive correlation (r=0.694, p<0.01), while autocratic leadership demonstrated a weak negative correlation (r=- 0.267, p<0.05). Transformational leadership accounted for 59% of the variance in teachers' content mastery, indicating that inspirational and intellectually stimulating leadership approaches significantly enhanced subject matter knowledge, pedagogical content knowledge, and curriculum knowledge among teachers. Transformational and democratic leadership styles significantly influenced teachers' content mastery, while autocratic leadership negatively impacted teachers' subject knowledge development. Headteachers who inspired continuous learning, provided intellectual stimulation, and created supportive professional development environments promoted higher levels of content mastery among teachers. The Ministry of Education and Sports should prioritize leadership training emphasizing transformational and democratic approaches, establish continuous professional development programs focusing on subject content knowledge, create mentorship systems pairing expert teachers with those requiring content strengthening, and allocate adequate resources for teachers' further academic and professional advancement in their subject areas.