Mentoring programs hold great promise for fostering competency in disadvantaged youth. Although considerable theoretical work has been conducted to explain the role of mentoring relationships in promoting positive youth outcomes, very little empirical research has directly investigated this alliance. The present study developed and validated a tool to assess mentees' perceptions of their relationships with their mentors and to investigate the relationship between this alliance and youth competency. Based on pilot data from four cross-national mentoring programs (N=276), a 10-item, two-factor Mentor-Youth Alliance Scale (MYAS) was created. Subsequent confirmatory factor analyses conducted on national evaluation data from a multi-site study of mentoring programs (N=219), generated a 10-item, one-factor solution. The one-factor MYAS significantly predicted youths' scores in four competency domains: Family Bonding, Relationships with Adults, School Bonding, and Life Skills, at 8-months post-intake. Higher competency youth were more likely to be female, younger on average, and have a higher quality of relationships with their mentors than were lower competency youth. Implications for the development of the mentor-protégé relationship are discussed.