Previous research suggests that school-based mentoring programs like those offered by Big Brothers Big Sisters of America (BBBSA) yield small but statistically significant improvements in the academic performance of mentored students and in their beliefs in their own scholastic efficacy. The present study uses data from a randomized control trial involving over 1,000 students from 71 schools across the country to investigate further the academic benefits of school-based mentoring, and to enrich the field’s understanding of how schools can use volunteers to support students. We employ instrumental variables and other approaches to provide insight into why the BBBSA school-based mentoring program is effective, finding that the relationship between mentor and protégé appears to play a key role. The evidence suggests that developing a close relationship with a mentor led to better academic outcomes for students; in contrast, students who were mentored but did not experience a close relationship showed no improvement in academic outcomes relative to the control group. This pattern holds for mentoring relationships of various durations. In addition, there is no evidence that mentoring programs with an academic focus produced better academic outcomes than relationship-only programs. Findings do reveal, however, that programs structured with weekly meetings and with opportunities for pairs to interact outside of a large-group setting were more likely to generate close mentor-protégé relationships. Beyond reporting new empirical findings, this paper contributes a theoretical structure with which to assess the results of randomized evaluations of mentoring programs.