Preparing teachers to provide an equitable and socially just education has become a focus for many teacher preparation programs across the country. Yet, relatively little research attention has been given to the role of mentor teachers--the people who student teachers identify as most influential in their learning to teach--in this work. This study examined White mentor teachers' shifts in thinking and actions during a particular professional learning opportunity--a Mentor Study Group focused on mentoring for social justice. Findings demonstrate both the importance of this work and a path forward for doing this work in the context of a teacher preparation program.