A randomized control trial involving 806 youth (ages 10-16; 85.4% low-income households) served in U.S. Big Brothers Big Sisters affiliates investigated effects of incorporating activities to promote youth thriving into mentoring relationships over a 15-month period. Outcomes included support for thriving in youths’ relationships with adults, youths’ personal resources for thriving, and levels of problem behavior. Intent-to-treat analyses revealed no differences in outcomes based on assignment to thriving promotion or standard services. There was substantial variability in youth exposure to thriving promotion activities, primarily in association with premature endings of mentoring relationships. In path analyses, positive engagement with the activities predicted enhanced support for thriving from adults and, via this support, increased personal resources for thriving and reduced problem behavior.