Big Brothers Big Sisters (BBBS) facilitates mentoring relationships between youth and volunteer mentors. Although research has examined outcomes for youth in BBBS, relatively less investigation has been undertaken for volunteer outcomes. This study explored factors associated with changes in psychological well-being among BBBS volunteer mentors. Participants included 593 mentors (Mage = 31) surveyed at study baseline and 15-month follow-up. A classification and regression decision tree approach was used to predict residualized change in psychological well-being from study baseline with match length included as the first split variable, and demographic, individual, and relationship variables included as candidate predictors. Analyses indicated that mentors with longer relationships (>4.5 months) reported more positive change in psychological well-being compared with mentors with shorter relationships. Perceived quality of program supervision was a further predictor within both groups of volunteers. Findings suggest that longer relationships and greater program support may contribute to mentor well-being.