This study examines youth initiated mentoring (YIM), a new approach to mentoring in which youth nominate mentors from among the non-parental adults within their existing social networks (e.g., teachers, family friends, extended family members). YIM is currently being implemented through the National Guard Youth ChalleNGe Program (NGYCP), an intensive residential intervention program for youth ages 16-18 who have dropped out or been expelled from high school. This study employed a mixed methods explanatory design, drawing on quantitative data from a national longitudinal evaluation of NGYCP (N = 1,173) and qualitative data from a subsample of participants (N = 30) in the evaluation. Results indicated that more enduring mentoring relationships were associated with increased retention of educational, vocational, and behavioral outcomes 3 years following entry into the study. Qualitative data suggested that, when relationships endured, mentors contributed to improvements in participants' educational and occupational success, quality of relationships with parents, peers, and others, and self-concept by providing social-emotional support, instrumental support, and guidance. Results also revealed that relationships were more likely to endure when youth chose their mentors on their own (rather than receiving help from parents or program staff) and when mentors were of the same race as youth. Implications for research and practice are discussed.