Principal preparation programs use various components to develop candidates, including coursework, core assessments, and field-based internships or practicums. The internship represents an exceptionally high leverage learning opportunity (Lochmiller, 2014). The success of the internship depends highly on the abilities of a mentor via informal instructional supervision. Mette (2020) explains that supervision has a potential transformational quality. This study explores mentorships in a university principal preparation program through the mentor's perspective. We interviewed a sample of mentors nominated as effective by peer and program informants. We learned that mentors were deliberate in developing three specific skills --metacognitive ability, developmental efficacy, and learning-oriented goal setting. Mentor principals described these three elements as paramount to master before becoming a principal. We discuss implications for principal preparation programs to use mentors as informal instructional supervisors and develop highly effective school principals.