Previous research demonstrates that social and interpersonal factors, more than academic preparation, affect decisions by under-represented students to stay in or to leave STEM fields. Yet, much of the theorizing about STEM learning in higher education begins with conceptual and epistemological dimensions. We make the case for a new theoretical framework, a "learning humanities," that begins with relationships. From this relational starting point, we locate STEM knowers as actors in relationships who become "answerable" for their STEM knowledge and take "wise actions" from this place. We then use this framework to analyze learning for STEM undergraduates involved in the STUDIO: Build Our World program, an afterschool mentoring program for low-income, immigrant, and refugee youth of color. Drawing on narrative and ethnographic analyses of data from 12 focal mentors, we found that mentors developed 3 focal practices identified by Edwards, "relational expertise," "common knowledge," and "relational agency" through their efforts to create the best possible program for youth. This built mentors' sense of answerability and a capacity for wise action within and outside of STUDIO. We argue that this theoretical stance provides new ways to conceptualize the nature, purpose, and outcomes of STEM learning for historically non-dominant STEM undergraduates' learning.