Research suggests that teacher agency influences different areas of English as a foreign language (EFL) teachers’ practice and professionalism. Nevertheless, the role of teacher professional agency in facilitating sustainable professional development (SPD) and work engagement has been overlooked. To fill this gap, this study used a semi‐structured interview with a sample of 27 Chinese EFL teachers to unveil their perceptions of the interplay of concern. The results indicated different venues through which EFL teachers’ professional agency could contribute to their SPD and work engagement. Professional agency could lead to teachers’ SPD by affecting their teaching motivation, positive emotional experiences, professional enthusiasm, and self‐development. Conversely, it was found that professional agency facilitated EFL teachers’ work engagement by influencing their teaching methods/materials, enthusiasm, satisfaction, commitment, responsibility, and teaching horizons. The findings are discussed and implications are listed for EFL teachers to use agentic practices in second/foreign language (L2) education, as triggers of many other positive outcomes like engagement and professional development.