In today's educational system, teachers are highly accountable for implementing policy-driven changes in student learning and the development of the quality of educational institutions. The present study aimed to expand this research area by investigating the contributors as well as the manifestations of teachers' professional agency. To this aim, first, ten EFL teachers were selected based on convenience sampling from ten high schools from Mashhad, Iran. Data were collected through semi-structured interviews. The researchers recorded, transcribed, and analyzed interview data. Regarding the contributors to EFL teachers' professional agency, the participants' responses were classified into three main groups, including authority-related factors, teacher-related factors, and learner-related factors; each group is composed of two or more sub-factors. Concerning authority-related factors, the participants' responses indicated that the principal's trust and support played important roles in extending the teachers' professional agency. Respecting teacher-related factors, teachers' experience of pre-service education in line with their professional competencies and motivational attributes (motivation, goal orientation) were found significant in constructing professional agency. Regarding learner-related factors, learners' willingness toward teachers' innovative activities was highlighted in shaping teachers' professional agency. Moreover, the results proved that professional agency was manifested mainly at three levels, namely curriculum designing, school level, and classroom level. Finally, implications were provided in the realm of teacher education and teacher practitioners.