Although the importance of mentor teachers in clinical teacher preparation is well established, few researchers explore the social identity development of these individuals. Through our study we contribute to the body of research by exploring mentor teachers' social identity development through the concept of Apprenticeship of Observation -- specifically, how they felt their own mentoring experiences influenced their approaches to mentoring. The multi-case study includes findings about mentoring beliefs and practices during the laboratory school component of an Alternate Route to Licensure program. Incorporating semi-structured interviews and video analysis, the findings demonstrate how four mentor teachers' prior experiences as mentees -- including Alternate Route to Licensure, traditional teacher preparation programs, and inservice teaching -- influenced their interactions with teacher candidates as mentors. Recommendations for practice and implications for future research are provided.