The goal of this project was to evaluate the effects of an innovative intervention model created specifically for youth in alternative school settings. The Motivational Interviewing with At-Risk Students (MARS) Mentoring Program is a unique intervention rooted in self-determination and behavior modification theories and delivered through a motivational interviewing framework. Student participants were enrolled at a K-12 district disciplinary alternative school and were predominately Black males who qualified for free or reduced price lunch (N = 39). Students were randomized using a block design to receive MARS Mentoring or to a wait-list control group. Regression analyses revealed significant and large treatment effects at 3-month follow-up. Mean improvements on the Behavior Assessment System for Children-Third Edition (BASC-3) social, emotional, and behavioral composites all indicated medium to large effect sizes. Students enrolled in MARS also experienced significantly fewer school disciplinary actions and improved academic performance. Analysis of intervention feasibility and acceptability reports indicated the program to be well received by stakeholders and practical to implement within the school setting. This brief, manualized intervention can be readily disseminated and has the potential to serve as a model for addressing the unique and profound social, emotional, and behavioral needs of youth in alternative school settings.