To improve educational outcomes through improvements in teaching standards, the Ministry of Education has stepped up efforts aimed at providing continuous and lifelong learning among Ghanaian teachers. However, the impact of the recent emphasis on a comprehensive framework for the professional development of teachers on teaching and learning outcomes has not been adequately studied. Despite the growing body of literature suggesting that professional development and professional knowledge may be associated with student outcomes, these relationships remain an open question. This study, therefore, sought to determine the impact of teachers’ professional development on academic performance in senior high schools in Ghana and the mediating role of teachers’ professional knowledge. A survey was conducted, and valid questionnaires were retrieved from 4,102 teachers in selected public senior high schools from four regions (including 77 districts and 97 public senior high schools) across the Southern, Middle, and Northern belts of Ghana. The results of the study showed that professional development has a significant positive relationship with professional knowledge (γ = 0.26, p < 0.001). Also, professional development and professional knowledge were positively related to academic performance (γ = 0.08, p < 0.001) and (γ = 0.04, p < 0.001) respectively. Finally, the results of the study showed that professional knowledge partially mediated the relationship between professional development and academic performance. The study, therefore, concluded that professional development leads to improvements in the academic performance of students through improvements in teachers’ professional knowledge and other characteristics of teachers that were not accounted for by this study. Among other things, the study recommends that teachers should be encouraged to engage in diverse professional development activities that respond to the inadequacies in their professional competencies.