This study investigates how the implementation of program-level practices by formal youth mentoring programs is associated with the quality of youth mentoring relationships as contexts for youth development and also examines whether this connection is mediated by the mentor-staff working alliance. Using data from mentors (n = 542) participating in multiple programs (n = 55), multilevel path models examined hypothesized direct and mediated effects. Parallel analyses were conducted with assessments of program practices from staff (n = 219). Greater exposure to program practices was associated with higher ratings of mentoring relationship satisfaction, commitment, and security and lower mentor-youth relationship negativity. The mentor-staff working alliance either partially or fully mediated these associations. Staff-reported practices predicted mentoring relationship satisfaction and commitment without mediation by the working alliance. This study suggests program practices contribute to stronger youth mentoring relationships. The findings also highlight the mentor-staff working alliance in supporting the development of positive mentoring relationships.