This study explores the pathways through which school-based mentoring relationships are associated with improvements in elementary and high school students' socio-emotional, academic, and behavioral outcomes. Participants in the study (N = 526) were part of a national evaluation of the Big Brothers Big Sisters school-based mentoring programs, all of whom had been randomly assigned to receive mentoring at their schools over the course of one academic year. Students were assessed at the beginning and end of the school year. The results of structural equation modeling showed that mentoring relationship quality, as measured by the Youth-Centered Relationship scale and the Youth's Emotional Engagement scale, was significantly associated with positive changes in youths' relationships with parents and teachers, as measured by subscales of the Inventory of Parent and Peer Attachment, the Teacher Relationship Quality scale, and the Hemingway Measure of Adolescent Connectedness. Higher quality relationships with parents and teachers, in turn, were significantly associated with better youth outcomes, including self-esteem, academic attitudes, prosocial behaviors, and misconduct. The effect sizes of the associations ranged from 0.12 to 0.52. Mediation analysis found that mentoring relationship quality was indirectly associated with some of the outcomes through its association with improved parent and teacher relationships. Implications of the findings for theory and research are discussed. (Contains 3 tables and 3 figures.)