The clinical development of nurse practitioners (NPs) has historically been dependent on mentorship from medical practitioners, yet their experience of this mentorship is generally unexplored. NPs have an ambiguous relationship with medicine as they have been dependent on medical mentorship to develop clinical skills, and they substitute into roles traditionally associated with medical practice. Consequently, NPs challenge professional boundaries and present particular concerns to their medical mentors. Practitioner ethnography examined the experiences of medical mentors, nurse practitioner students and academic staff during a clinical degree programme. This paper reports specifically on the medical mentors, focusing primarily on their professional authority relationship with their students and on their experience of imparting and sharing clinical knowledge. These experiences fell into three perspective stages, the provisional perspective, transitional perspective, and final perspective. Medical mentors were instrumental to the advanced clinical role of the student NP. This resulted in a conflicting experience of promoting a clinical role that challenged traditional medical authority. The effect of this was a cautious renegotiation of professional boundaries. In future NP students (and their academic teachers) need to acknowledge this if they are to mutually gain the most from their relationship with their medical mentors.