Professional development (PD) opportunities for teachers most commonly take place outside the classroom and are typically designed to address specific components of teachers' instruction or curriculum. However, there are other activities that take place within the classroom that may have a profound impact on the PD of a teacher. We sought to gain a deeper understanding of the influence that placing a teacher in a mentor role can have on their PD and practice. Thus, we researched the influence of mentor teachers' work with university-level STEM education majors by engaging in teaching a limited series of STEM inquiry-based lessons in the mentors' classrooms. Surveys of the mentor teachers indicated that there were many positive benefits for mentors, including gaining new ideas, increased reflection on their practice, increased engagement of students, and in some cases shifts in practice.