Background: Although early closure of formal youth mentoring relationships has recently begun to receive some attention, more information about factors that contribute to premature endings, and how those factors interact, is needed so that empirically-based program practices can be developed and disseminated to prevent such endings and to ensure that youth reap the benefits mentoring can provide. Objective: This qualitative interview study applies a systemic model of youth mentoring relationships (Keller in J Prim Prev 26:169-188, 2005a) to the study of mentoring relationship endings in community-based mentoring matches to understand why these matches ended. Method: Mentors, parents/guardians and program staff associated with 36 mentoring matches that had ended were interviewed about their experiences of these relationships and their understanding of why they had ended. Thematic analysis of the interview transcripts and mentoring program case notes for each match followed by systemic modeling of the relationships yielded three major findings. Results: A strong mentor-youth relationship is necessary but not sufficient for match longevity. The mentor-youth relationship, even when relatively strong, is unlikely to withstand disruptions in other relationships in the system. Agency contextual factors, such as program practices and policies and staffing patterns, have a critical role to play in sustaining mentoring matches, as they directly influence all of the relationships in the mentoring system. Conclusion: These findings highlight the importance of considering not just the mentoring dyad but also the parent/guardian and program context when trying to prevent match closures. They also point to several program practices that may support longer mentoring relationships.