Teacher education is in the midst of a national shift toward clinically centered teacher preparation, answering recent calls from professional associations, scholarly and policy reports, and accreditation agencies. This model of educator preparation centers efforts in clinical or school-based settings--rather than in university-based contexts--while also purposefully intersecting teacher candidates' opportunities for academic learning and professional applications of pedagogical skills. It is within this context that the roles school-based teacher educators play in clinical teacher preparation are gaining increased attention. As the key link between learning theory and implementation of pedagogical strategies, mentor teachers are recognized by university-based teacher educators as "experts of practice." Yet the roles of the mentor teacher in teacher education endeavors--and specifically their preparation for such essential and valued roles--are largely unexplored. In this article, the authors outline the first year of their now three-year effort to respond to these traditions and assumptions by codesigning a cost-effective and sustainable approach to training PK-12 mentor teachers to serve as boundary-spanning, school-based teacher educators. They also describe participants' developing perspectives on mentoring and their assessments of the impacts of the training based on an examination of the first-year implementation. They conclude with a consideration of the implications of these initial training efforts for their own contexts, others' settings, and the broader field of educator preparation.