Mentoring relationships are among the most crucial forms of developmental support for students in higher education. While mentorship has been widely studied, researchers' conceptualizations and frameworks of mentoring relationships have scarcely been updated with notions of equity-mindedness to meet the needs of today's increasingly diverse student population. Using qualitative data from 10 doctoral students serving as mentors to undergraduates in STEMM, this study examines the strengths and shortcomings of Kram's model about the phases of mentoring relationships. By extending four decades of research on these well-established phases of mentoring (i.e., initiation, cultivation, separation, redefinition) with understandings from the equity-minded mentoring model, findings illustrate how the phases of graduate-undergraduate mentoring are shaped by mentors' social identities and larger organizational dynamics. Collectively, study findings illuminate new empirical understandings about STEMM graduate students' roles as mentors and move conceptualizations about relational mentoring phases toward equity-minded understandings.