This study examined a model of the antecedents and outcomes of mentor commitment to workplace mentoring relationships. The proposed model was based on the investment model of commitment. A total of 180 pairs of mentors and their protégés completed surveys that assessed model constructs. Results indicated that mentor relationship satisfaction and investment size predict mentor commitment, whereas mentor quality of alternatives and perceptions of managerial support for mentoring do not predict mentor commitment. Additionally, mentor commitment is associated with information exchange behaviors engaged in by mentors and protégés. These findings suggest that commitment plays an important role in mentoring relationships, and the investment model provides a useful framework for future research on this topic.