While past researchers suggest undergraduate peer mentors (PMs) benefit from mentoring their peers, this experience is rarely associated with transformative learning. Using narrative analysis of authentic mentoring stories, we explored how particular types of mentoring experiences contribute to transformative learning for PMs of first-year university students. In this study, transformation was more likely when PMs engaged in meaningful routines and everyday practices; exercised purposeful "pretending" in unfamiliar aspects of their role; embraced challenge and surprise; regularly reflected on experiences; and were mentored by supportive faculty and staff. Findings have implications for PM selection and training, as well as program design.