Preliminary analyses conducted on a pilot sample generated the Mentor–Youth Alliance Score (MYAS), a 10-item, 2-factor tool, with Cronbach’s internal consistency coefficients ranging from .73 to .85. There were 10 positively worded items assessing youths’ perceptions of the relationship. Moderately strong relationships emerged between participants’ total and subscale MYAS scores and their scores on an instrument that assessed the quality of their relationships with adults. Subsequent confirmatory factor analyses performed on the 10-item, 1-factor solution, with a Cronbach’s internal consistency coefficient of .92. To determine the tool’s predictive validity, the one factor solution was submitted to variable-based competency analyses. Results indicated that the 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. Data for Study 1 were collected from 276 youth participating in a mentoring program; data for Study 2 were collected from 219 youth participating in a substance abuse prevention program for at-risk youth through monitoring services. Tables and references