This study explored how ESL and content teachers’ collaboration influenced content teachers’ learning to plan for and teach ESL students in mainstream content classrooms and ESL students’ participation during the collaborative teaching sessions. This study is theoretically based on sociocultural learning notions that assume that teacher learning is a dynamic and complex process that occurs through teachers’ professional interactions (Johnson & Golombek, 2016) in communities of practice (Lave & Wenger, 1991). This study uses positioning theory (Davies & Harré, 1990; Harré & Moghaddam, 2003; Harré & van Langenhove, 1999) as its theoretical lens, which affords the opportunity to investigate how reflexive and interactive positionings (Davies & Harré, 1990) shape the possibilities for content teachers’ learning to plan for and teach ESL students and for ESL students’ participation during the collaborative teaching sessions in mainstream content classrooms. Employing a case study research design, this study assumes the researcher’s dual participatory role as the ESL teacher. In doing so, I collaborated with four content teachers separately to show how collaboration influenced content teachers’ learning to plan for and teach ESL students and ESL students’ participation in the mainstream content classrooms. Data collection included qualitative methods, such as semi-structured interviews, collaborative planning sessions, lesson planning artifacts, collaborative teaching sessions, collaborative viewing sessions, reflective journals, ESL students’ work samples, and field notes. Data analysis relied on Saldaña’s (2013) coding techniques and drew on Davies & Harré’s (1990) reflexive and interactive positionings. This study found that ESL and content teachers’ collaboration influenced the content teachers’ learning to plan and teach ESL students because collaboration became a space for teachers to learn by negotiating lesson designs, teaching roles, language strategies, assessment techniques, and the ESL teacher’s role in the mainstream content classrooms. This study also found that collaboration influenced ESL students’ participation by creating space for both the ESL teacher’s increased role and the content teacher’s renewed student positionings. In addition, it shed light on how teachers’ collaborative planning and teaching acts enhanced or constrained ESL students’ participation. In light of the findings, the study calls for increased collaborative efforts between ESL and content teachers.