Youth mentoring relationships have significant potential for promoting positive youth development. Nonetheless, the benefits derived from such relationships depend considerably on the length and quality of the bonds that are created between mentors and youth. Although some attention has been paid to youth’s experience of relationship quality, few studies have focused on mentors’ experience of relationship quality. In the context of a national sample of mentor and youth dyads in Big Brothers Big Sisters community-based mentoring programs (N = 5,222), the current study validated a new mentor-reported measure of relationship quality, explored associations between mentor and youth assessments of relationship quality, and investigated the capacity of early assessments of relationship quality to predict mentoring relationship duration. Implications for research and practice are discussed.