The ability to make informed decisions through teachers’ participation is the best approach to success in schools’ academic performance. Unfortunately, school heads find it difficult to take this approach in their schools in enhancing teachers’ job and students’ academic performance. Several informative studies have been carried out across the globe about what influences teachers’ job commitment. However, little is known about the impact of teachers’ participation in decision making on job performance, this is also the case in public schools. For this reason, this study investigated the impact of teachers’ participation in decision making on their job performance in public secondary schools in Tanzania. It specifically investigated the contribution of teachers’ participation in decision making, the challenges facing teachers’ participation in decision making and possible measures to ensure effective decision making in enhancing academic performance in public secondary schools. The study employed a mixed research approach that adopted a convergent parallel design. Ninety six (96) respondents were involved, including; ninety two (92) teachers and four (04) heads of schools. Questionnaires and semi-structured interviews were used to collect data. Data were analyzed through descriptive statistics with the help of SPSS version 20 and thematic analysis. The findings indicate that, teachers’ participation in decision making (TPDM) has a positive influence on their job performance; however, the level of teachers’ participation in decision making varies within and among schools. The study concludes that, heads of schools should make sure that all teachers are fully involved in decision making. The ministry of education should promote and organize training on participative management for teachers and school leaders through various professional development programs in order to enhance teacher’s job performance and student’s academic performance. Furthermore, the head of schools should build a relationship that fosters trust among teachers themselves and the school management.