To examine associations between White mentors’ beliefs regarding the presence of discrimination towards Black, Indigenous, and people of Color (BIPOC) individuals and mentoring relationship outcomes, mentors’ beliefs about racial/ethnic discrimination were assessed before random mentee assignment and at the end of 9 months of mentoring. White mentors matched with BIPOC youth showed greater increases in beliefs that discrimination limits opportunities for Black Americans. Stronger endorsement of the impacts of discrimination for Hispanic Americans resulted in less youth relationship anxiety when White mentors were matched with White mentees, but not when they were matched with BIPOC mentees. Last, greater increases in beliefs that discrimination limits opportunities for Black Americans resulted in less relationship anxiety for White mentors matched with White mentees, but more relationship anxiety for those matched with BIPOC mentees. Programs should assess and address mentors’ racial biases to minimize harm and augment the impact of mentoring programs for all youth. Highlights White mentors may enter programs without understanding the discrimination their BIPOC mentees face. White mentors’ racial biases shift over time, and can predict mentoring relationship quality. Mentoring programs should assess mentors’ racial biases to minimize harm for BIPOC youth.