There is growing recognition about the importance of studying teacher agency in working with Linguistically Diverse Students (LDSs) after No Child Left Behind (NCLB) and Every Student Succeed Act (ESSA) given that the high-stakes testing and accountability system, required by NCLB and largely maintained by ESSA, have exerted negative effects on LDS educators and students. In response to this, we compiled a comprehensive set of empirical studies and conducted a thematic synthesis to examine how various factors could impact pre-service and in-service teachers' agency in English as Second Language (ESL) and bilingual PreK-12 classrooms in the U.S. We adopted the ecological model of teacher agency as the analytical framework to guide our synthesis. Through both a top-down and bottom-up coding process, we further expanded the existing ecological model by adding two factors: knowledge and emotions. Detailed analyses led to seven common factors that could shape teachers' agentic power in working with LDSs, all of which are either from the iterational or practical-evaluative dimension of agency. The projective dimension of teacher agency is relatively less discussed. Among those factors, cultural and structural factors draw most research attention across the selected studies, with the structural factors exerting most constraining effects on teacher agency. Additionally, we uncovered nuanced differences in teacher knowledge and emotions through in-depth analysis of the ESL and bilingual contexts. Further implications on how to improve teacher education and professional development are provided, in addition to the future directions for research.