English-medium instruction (henceforth EMI) is a growing global phenomenon in education. The prevalence of EMI reflects a big shift to the approach of language teaching (Shohamy 2013). Local educational contexts (public and private schools, institutions etc.) in different countries have implemented EMI in their own ways with different goals, features and pedagogies. However, how teachers in different teaching contexts enact an English as a lingua franca (henceforth, ELF)-informed EMI paradigm remains largely unexplored. Some questions come to mind: How do local teachers enact an ELF-informed EMI curriculum? How does an ELF-informed EMI paradigm shape classroom instructions? In this chapter, I narrate my professional development in switching to an ELF-informed EMI teaching paradigm. My inquiry on my professional development takes the form of autoethnography (Ellis (2009). I adopted an analytical autoethnographic approach to illuminate how an ELF-informed EMI teaching paradigm affected my professional identity as a legitimate English teacher in a Japanese university. In my ethnographic account, I engage with texts and literature on ELF, EMI and teacher identity to explain my classroom instructions. I also critically reflect on the challenges I experienced in an ELF-informed EMI teaching paradigm.