In this article I examined the professional identity development of five mentor teachers in a year-long, clinically rich teacher residency partnered between a university-based teacher education program and schools in a southern state of the United States. Qualitative data were collected through classroom observation and individual semi-structured interviews with a focus on participants' mentoring activities and the ways they enacted and described their identities. Participants came to new mentoring beliefs and practices as they navigated the residency and developed a multifaceted identity to mediate their learning to become mentors and teacher educators. Implications for mentor teacher professional support, teacher preparation, and future research were discussed.