Few researchers have explicitly examined the benefits to practicing principals engaged in mentoring aspiring principals. Through the lens of transformative learning, the purpose of our study was to examine the professional learning of practicing principals engaged as mentors with aspiring principals in a leadership preparation program. Two questions guided our study. First, how does engaging in the mentoring process lead to professional learning? Second, how does that learning manifest itself in principal practice? Findings suggested principal mentor/protégé and mentor/mentor interactions prompted mentors to question their own experiences and practices, problematizing some of these practices. Principals then generated solutions, which lead to changes in core beliefs and assumptions and finally changes in practices, reflective of transformative learning.