Few studies have examined the impact that mentoring (i.e., developing a special relationship with a non-parental adult) has on educational achievement and attainment in the general population. In addition, prior research has yet to clarify the extent to which mentoring relationships reduce inequality by enabling disadvantaged youth to compensate for a lack of social resources or promote inequality by serving as a complementary resource for advantaged youth. Results from a nationally representative sample of youth show (1) a powerful net influence of mentors on the educational success of youth and (2) how social background, parental, peer, and personal resources condition the formation and effectiveness of mentoring relationships. The findings uncover an interesting paradox-that informal mentors may simultaneously represent compensatory and complementary resources. Youth with many resources are more likely than other young people to have mentors, but those with few resources are likely to benefit more from having a mentor-particularly teacher mentors-in their lives.