Youth mentoring programs have grown in popularity, both within the United States (U.S.) and abroad, as an intervention to support youth with common behavioral and emotional difficulties. However, it is unclear whether certain dimensions of youth risk may diminish the positive impact of formalized mentoring relationships. The current study therefore examined whether youth emotion regulation, a transdiagnostic risk factor for both externalizing and internalizing behavioral difficulties, predicted mentoring relationship quality and the likelihood of early match closure. Participants included 1,298 randomized mentor-youth dyads from two nationwide mentoring programs, one with chapters across the U.S. (youth: 56% female; 37% White), and another with chapters across Mexico (youth: 49% female; 100% non-Indigenous). At baseline, youth completed the Emotion Regulation Questionnaire for Children and Adolescents (ERQ-CA). At program completion, youth and mentors completed measures of mentoring relationship quality. Multigroup structural equation models of youth outcomes revealed that greater youth use of cognitive reappraisal predicted better mentoring relationship quality in both countries when co-varying for sex, and that this relationship was stronger for mentor-youth pairs in the U.S. compared to those in Mexico. These findings have important implications for understanding the ways in which youth characteristics might shape the quality and impact of mentoring relationships across different cultural settings.