Mentor training on cultural humility is an area of needed support in formal youth mentoring relationships. This pilot study used an experimental design to examine the role of a social justice and race equity training on volunteer mentors' cognitive and affective outcomes related to cultural humility in mentoring. The sample included 99 volunteer mentors paired with adolescent mentees in an established formal mentoring program. Mentors predominantly identified as White (89%), and the majority (72%) were paired with youth of color. Participants were randomly assigned to either the training or control condition. Findings from intention-to-treat analyses indicated that training group participants (n = 49) exhibited greater increases in self-efficacy to provide racial/ethnic support over time than participants in the control group (n = 50). As-treated analyses indicated that training attendees (n = 23) exhibited greater increases in self-efficacy to provide racial/ethnic support over time than participants who did not attend the training (n = 76). Results indicated no significant changes over time in participants' training content knowledge, awareness of racial privilege, ethnocultural empathy, or social justice interest and behavioral intentions. Analyses also indicated an attendance bias within the training condition, such that mentors who attended the training reported significantly more awareness of racial privilege, social justice interest, and social justice behavioral intentions compared to training condition mentors who were invited but did not attend the training. Implications for training volunteer mentors within formal mentoring programs are discussed.