Formal mentoring programs rely on mentors to build supportive relationships with youth with the intention of providing positive developmental opportunities for the youth. This dissertation, which includes three studies, explores the experiences of mentors, focusing on factors that contribute to mentors committing to and sustaining mentoring relationships, and how mentors approach building a supportive relationship. Study 1 develops and tests a conceptual model of mentor retention integrating concepts from the volunteerism and interpersonal relationship literatures to predict mentor retention. Participants were 51 college student-mentors. Path analysis showed support for the conceptual model. Mentor retention was predicted by their commitment but not by role identity. Role identity was positively related to mentor commitment. Role identity was predicted by relationship satisfaction, available alternatives and investment; only satisfaction predicted commitment. Study 2 examined how program practices influence mentor commitment utilizing secondary data from 551 mentors from mentoring programs involved in a large, randomized controlled trial evaluating an intervention to improve mentoring program quality. Path analysis demonstrated that how well the mentor felt the program set expectations and whether they were matched with a youth based on their preferences were associated with the mentor’s commitment. The relationships between commitment and program practices were partially mediated by the mentor’s relationship satisfaction and available alternatives. Study 3 explores how mentors approach building their mentoring relationship, whether different approaches contribute to supports offered to the youth, and if there are differences by gender or whether the mentor has previous experience mentoring. Thematic analysis of 16 mentoring relationships did not identify any specific approach as best. Instead, what mattered was the fit of the approach with the specific circumstances of the match. Mentors whose approach fit well or who were able to adapt their approach had the longest matches and provided the most support. Males were more often described to have an approach that was a good fit and to have provided more support. Whether previous mentoring experience helped a mentor build the relationship and support the youth depended on how the previous relationship went and how it influenced the mentor’s expectations with regard to the current match.