Purpose: A growing body of research has pointed to the potential benefits of a co-teaching clinical residency model in preservice education. Preservice co-teaching research has focused primarily on conditions necessary for effective co-teaching to occur, factors that inhibit successful co-teaching implementation, and teacher candidate development. Researchers have called for further exploration into potential benefits of preservice co-teaching models for the mentor teacher. In this study, the authors explored ways in which mentor teachers who participated in a co-teaching pre-service program experienced professional growth. Design/methodology/approach: In order to gain insights into the perspectives of mentor teachers and ways in which they experienced professional growth through their experiences in the co-teaching program, the authors used a qualitative, descriptive approach. The authors' primary data source included interviews conducted with 42 mentor teachers from five content-areas. Researcher communication and interactions with co-teachers over time, along with artifacts including field notes, co-teacher reflections on practice, and program documents, served as peripheral data sources. Findings: Results indicated that co-teachers experienced meaningful professional growth in areas represented by the following themes: (1) critical reflection, (2) pedagogical renewal, (3) "in situ" feedback and refining practice and (4) application of learning to leadership roles. Originality/value: This study, which is one of only a few studies focusing explicitly on mentor co-teacher professional growth, provided new insights into learning opportunities afforded to mentor teachers through a participation in a blended model of co-teaching and communities of practice.