Introduction: The use of alcohol, tobacco, and other drugs (ATOD) is a leading cause of preventable morbidity and mortality among adolescents. While traditional interventions have targeted specific health-risk behaviors (e.g., substance use, initiation of sexual intercourse, truancy, etc.), the evidence suggests that using a positive youth development (PYD) framework may have positive impacts across a number of domains. Friday Night Live Mentoring (FNLM) is a PYD-based, cross-age peer mentoring program that engages teams of older high school-aged youth to mentor teams of middle school-aged youth in a structured, ongoing, one-on-one relationship. While studies have demonstrated significant but small effect sizes of intergenerational youth mentoring programs in which an adult mentor is paired with the youth mentee, research on cross-age mentoring programs is limited. The purpose of the current study is to evaluate FNLM on its ability to improve participants’ knowledge, attitudes, skills, opportunities to develop caring relationships, school engagement, and academic performance. Methods: A retrospective, pre–post survey was administered online to FNLM participants across 13 California counties. Participants rated their knowledge and attitudes about ATOD, skills, relationships with peers and adults, and academic indicators. Open-ended questions gathered information about participants’ experiences in FNLM. Non-parametric related-samples Wilcoxon signed rank tests (an alternative to paired t-test) were used to compare pre–post differences. Participants were also asked two open-ended questions: “What are the best parts of FNLM?” and “What, if anything, would you change?”. The responses to each question were reviewed, coded, and analyzed according to key themes. Results: A total of 512 participants completed the survey (287 mentors and 225 protégés). There were small but statistically significant improvements across all items for both mentors and protégés. Qualitative analyses showed that most mentors and protégés especially enjoyed getting to know and spend time with one another. Several mentors added that it was rewarding to be a positive influence on or to make a positive difference in the protégé’s life. Many youth stated that the relationships formed, especially with their partner, and the activities were the best part of FNLM. The overwhelming majority would not change anything about the program. Those who provided recommendations for program improvement suggested more activities or more hands-on and engaging activities and more or longer meetings. Conclusion: FNLM actively engages youth and provides them with support and opportunities that promote knowledge, skill development, positive relationships, academic engagement, and success and raise awareness of the harms that the use of alcohol, tobacco, and other drugs (ATOD) can cause. While ATOD use was low prior to program participation, it was significantly lower after participating in the program.