Teachers' participation in decision-making plays a significant in their work performance. In Tanzania, the idea of participation in decision-making emerged from decentralization policy and was adopted in the field of education through the Education and Training Policy of 1995. The theme was to emphasize that school administrators involve teachers in decision-making to improve school work performance. This study sought to find out the relationship between teachers' participation in decisionmaking and their work performance and identified the factors affecting teachers' participation in decision-making to work performance in secondary schools. Also, the study anchored on Transformational Leadership Theory developed by Mac Gregory Burns in 1978. Moreover, the study adopted a mixed research approach specifically convergent parallel to collate data from teachers, heads of schools, WEOs, and DEO for secondary schools. Validity and reliability of quantitative instruments were tested through content and appearance whereby 0.82 correlation coefficients were obtained to indicate the instruments were valid and reliable. Likewise, the reliability of qualitative tools was obtained through a triangulation system. Quantitative data were analyzed through a computer software program known as SPSS version 20 while qualitative data were analyzed through content analysis methods which involved assigning data into themes, categories, and summarization. The findings demonstrated that there is a relationship between teachers' participation in decision making as it increases efficiency, commitment, accountability and enhances students' performance. Also, the study indicated that teachers' participation in decision-making was affected by inadequate time for participation, financial challenges, facilities, transparency, and lack of leadership training. These factors led to poor academic performance in schools.