Graduate students and postdoctoral researchers (postgraduates) in the life sciences frequently mentor undergraduate researchers, especially at research universities. Yet there has been only modest investigation of this relationship from the postgraduate perspective. We conducted an exploratory study of the experiences of 32 postgraduate mentors from diverse institutions, life sciences disciplines, and types of research to examine their motivations for mentoring and their perceived outcomes. Although some postgraduates reported feeling pressured to mentor undergraduate researchers, all expressed personal motivations, including both agentic (self-focused) and communal (community-focused) motivations. These postgraduates reported benefits and costs of mentoring that had both vocational and psychosocial elements. Given that our results indicated that even postgraduates who engaged in mentoring at the request of their faculty advisors had their own motivations, we conducted a second phase of analysis to determine the extent to which our results aligned with different theories of motivation (self-determination theory, social cognitive career theory, expectancy-value theory, social exchange theory). We end by proposing a model of postgraduate mentoring of undergraduate researchers that integrates the theories supported by our findings.