Due to histories of maltreatment, living instability, and relational disruptions, youth in foster care are at increased risk for experiencing poorer well-being outcomes as compared to their non-foster peers. However, research suggests that the presence of a caring, supportive nonparental adult, such as a mentor, may function as a protective factor, offsetting some of the risk that these vulnerable youth face. Research identifies a positive association between mentored youth and improved psychosocial, behavioral, and academic outcomes, and greater effects are associated with higher quality mentoring relationships, leading researchers to investigate for whom and under what circumstances such relationships may be present. Among youth in foster care, both naturally occurring and programmatically matched, formal mentoring relationships have been investigated, though past studies have not explored how mentor type or relationship dynamics may explain variance in the quality of mentoring relationships for this population. Using survey data from 444 natural and formal mentors and interview data from 8 high and low scoring natural and formal mentors, this dissertation uses mixed methods to answer the research questions: To what extent do mentor type and relationship dynamics explain variance in the quality of the mentoring relationship for adolescent and emerging adult youth in and aging out of foster care, controlling for demographic characteristics? What barriers and facilitators of a quality mentoring relationship do natural and formal mentors of youth in foster care experience and identify? Findings from this study indicate that naturally occurring mentoring relationships were associated with longer mentoring relationships, whereas programmatically supported, formal mentors were associated with higher perceived efficacy. Internal dynamics of closeness and compatibility were positively associated with characteristics of quality relationships, such as longer relationships and more frequent and consistent contact. External dynamics, such as interference (i.e., personal/logistical stressors) decreased the length of the mentoring relationship. Finally, mentoring relationships among youth in foster care tended to benefit from a primary growth-focused component with an accompanying fun-focus. This study presents these findings and highlights future research and practice implications in order to promote quality-mentoring relationships among youth in foster care.