In the rapidly evolving landscape of education, the continuous professional development of teachers is essential for maintaining high standards of teaching and enhancing student achievement. In-service teacher training programs, which provide ongoing education and training for currently employed teachers, are a crucial component of this professional development. This study uses a mixed-methods approach to investigate the efficacy of in-service teacher training programs in enhancing student success and teaching quality. To obtain thorough data on their experiences and perspectives, 286 in-service teachers from various schools around Kampala completed a survey. Additionally, 52 in-service teachers were interviewed, providing a comprehensive insight into their experiences. According to the quantitative results, 71% of the respondents had previously taken part in in-service training; 61% reported improvements in the quality of their instruction, and 71% noted improvements in student performance. The chi-square tests and regression analysis results showed that in-service training significantly improved student achievement and teacher quality. These findings were corroborated by the qualitative data, which highlighted themes including improved teaching strategies, increased teacher confidence, enhanced student engagement, impact on student achievement, and teacher motivation and morale. The study also highlights the value of peer cooperation and hands-on workshops in training programs and emphasizes the necessity of ongoing, customized professional development. This study offers a comprehensive knowledge of the efficacy of in-service teacher training programs by integrating quantitative and qualitative data. This understanding will be beneficial for educators and policymakers as they build more effective professional development initiatives.