Shifting the existing lens of mentorship in graduate education to one that frames graduate students as mentors, rather than mentees, I explore how 10 STEMM doctoral student mentors considered social identities and organizational power structures in mentoring undergraduates. Using sociospatial and in-depth interview data, findings illuminate how mentors' approaches were characterized by identity concordance and discordance, knowledge of institutional and disciplinary minoritization, two-sided negotiation of organizational power dynamics, and organizational limits to providing tangible resources. This study documents individual and institutional complexities to fostering equity-minded mentoring, with crucial implications for training the next generation of STEMM faculty as equity-minded mentors.