Mentoring programs show promise for preventing emotional and behavioral problems among at-risk youth, but little is known about processes that may be most critical to achieving this end. This study explored indicators of positive youth development (PYD; competence, confidence, connection, care and compassion, character) as mediators of associations of mentoring support with youth emotional and behavioral problems. The sample included 501 youth participating in a study of the Big Brothers Big Sisters program in Canada (mean = 11.16 years old; 52% girls, and 44% White). Measures were youth self-report, with the exception of the use of both youth and parent report measures of emotional and behavioral problems. Results of structural equation modeling analyses were consistent with PYD mediating the association between mentoring support and emotional and behavioral problems, regardless of informant. The association between mentor support and PYD, however, was limited to youth in active mentoring relationships.