Formal community-based youth mentoring relationships (CBM) are a popular form of intervention worldwide in which caring, non-parental adult figures are matched with at-risk children (i.e., children who experience an intense and/or chronic risk factor, or a combination of risk factors in personal, environmental and/or relational domains that prevent them from pursuing and fulfilling their potential) to promote development and health. Common models suggest that a close mentoring relationship is needed for the success of the intervention. However, it remains unclear which key relational processes and variables promote relationship quality to generate the most significant benefits. Using the PRISMA framework, 123 articles were identified as relevant for this review which explores the state of the literature on CBM relationships describing the main findings regarding the characteristics of the relationship and the mediating and moderating variables. An essential ingredient that consistently emerged for generating mentoring outcomes is characterized by feelings of support, sensitivity, and trust and accompanied by a purposeful approach to shaping the goals of the relationship. A balanced approach comprised of recreational, emotional, and catalyzing aspects has been reported as essential for mentoring success. Mentors’ positive attitudes toward underprivileged youth, maturity in terms of age and experience are essential in forging positive relationships. Mentees who have better relational histories and more positive personality traits exhibited higher relationship quality. However, data imply the possibility of addressing mentees from moderate risk status. Preliminary evidence on thriving as a mediating variable was found. Program practices, such as training, parental involvement, and matching based on perceived similarities and similar interests, emerged as important factors. Generating many research suggestions, the review identifies research questions and uncharted territories that require inquiry.