Mentoring, particularly same-gender and same-race mentoring, is increasingly seen as a powerful method to attract and retain more women and racial minorities into science, technology, engineering, and mathematics (STEM) education and careers. This study examines elements of a mentoring dyad relationship (i.e., demographic and perceived similarity of values) that influenced the perceived quality of mentorship, as well as the effect of mentorship on STEM career commitment. A national sample of African American undergraduates majoring in STEM disciplines were surveyed in their senior year. Overall, perceived similarity, rather than demographic similarity of values, was the most important factor associated with protégé perceptions of high-quality mentorship, which in turn was associated with higher commitment to STEM careers. We discuss the implications for mentoring underrepresented students and broadening participation in STEM.