The focus on developing teacher expertise makes teaching and learning more sustainable, as it is a way of working to create improvement in education. The objective of this study was to explore the direct or indirect impacts of principal instructional leadership and school support on teacher expertise and explore the mediating effect of teachers’ professional development agency. A survey of 1123 teachers was conducted at 21 primary schools and 20 secondary schools in Hebei and Shanxi provinces of northern China. Structural equation modeling and bootstrapping were performed to test the relationships between variables. Results showed that teachers’ professional development agency mediated the effects of principal instructional leadership and school support on teacher expertise. School support was a better predictor of teacher expertise than principal instructional leadership. Providing instructional conditions and leadership support were non-significantly related to teacher expertise. Colleague support and student support were the better predictors of teacher expertise than providing instructional guidance and monitoring. The findings indicate that the growth of teacher expertise depends on building their professional development agency. Teachers will have a strong sense of agency to sustain the teaching profession when principals establish a supportive school climate that emphasizes teaching and learning in their leadership practice and enables teachers to build positive relationships with colleagues and students. The study confirms the supportive factors that impact teacher expertise and provides useful implications for the daily practice of teachers, principals, and administrators.