Reflective practice has become a mainstay in many inquiries into teaching and learning, presenting reflective practitioners with the challenge of accounting for their own institutional positions when interpreting student performance in the binary teacher-student configurations of most classrooms. This study analyzes the perspectives of TAs cast as mentors to students in a unique trinary configuration of instructor-mentor-student. During four semesters, TAs in English mentored first-year university composition students by attending all classes alongside them, conducting intake interviews, and following up with numerous out-of-class conferences during the semester. Using standardized end-of-term evaluations by mentors supplemented by focus group transcripts and administrators' field notes, analysts determined that mentors' ranges of actions in the classroom and course enabled them to "think through" the perspectives of both instructor and student to develop "positional reflexivity." That is, mentors incorporated the factor of institutional position into reflexivity about teaching and learning to gain insight into such issues as interpretations of student performance, power dynamics that inflect students' senses of agency, the challenges of transitioning to college, mentors' own professional goals, and more. Implications are drawn for leveraging this unique form of TA training to enhance learner-centered approaches to teaching when TAs later find themselves teaching their own courses.