Although research has clearly demonstrated the benefits of mentoring for mentees, much less is known about how mentoring impacts mentors and, in particular, their professional development. Using a mixed-methods design, this study investigated the impact that mentoring youth had on the professional development of first- and second-year university students, primarily preservice teachers. Researchers collected quantitative data from twenty-eight undergraduate university student mentors pre- and post-program, as well as twenty-nine ninth-grade mentees post-program. In addition, a focus group was conducted with six mentors post-program. The data analyses were guided by the Graduating Teacher Standards: Aotearoa New Zealand (GTS), which outline what a beginning teacher should know at the conclusion of a professional teacher education degree program. The results indicated that mentoring had a positive impact on mentors' development of the values, skills, relationships, and knowledge required to satisfy the GTS. The implications of these findings are discussed within the context of professional development. (Contains 3 tables.)