Social capital, including engagement with mentors, facilitates educational attainment. However, engagement with mentors differs significantly across groups of adolescents with different backgrounds, including an immigrant background. We investigate how immigrant generation predicts adolescents' engagement with mentors and different types of mentors (i.e., school-based and non-school-based), the association of mentors with educational attainment, and these estimates' heterogeneity based on the immigrant generation. We analyzed nationally representative Add Health data from N = 11,242 adolescents by using school fixed-effect linear probability models. Results show that adolescents from immigrant generations 1 and 2 were less likely than those from generation 3+ to have a mentor, but there were no significant differences in engaging with school-based mentors. Mentors predicted educational attainment; school-based mentor effects were larger than non-school-based mentor effects. The associations between mentors and college attendance and graduation were largest for first-generation immigrants. Our findings indicate the importance of structures supporting relationship-building and mentorship in schools and wider communities.