Education reform and promotion of adolescent positive health (both physical and psychological) are two major public policy issues that can be impacted by direct intervention. The fields of positive psychology and positive youth development offer a theoretical foundation for scientific research as well as pragmatic, evidence-based methods of positive interventions (PIs). Mentoring is a unique intervention method geared towards both promotion and prevention. The Youth Mentoring Partnership’s Friend Fitness Program represents a new PI method with the goal of strengthening character traits in young people through a unique form of physical fitness-based mentoring in which adolescents must persevere through intense “moments of choice.” This paper will present the theoretical foundations of and initial empirical support for the Friend Fitness Program’s efficacy based on a longitudinal assessment. First, the theoretical framework will be presented through a comprehensive literature review on positive youth development, positive interventions, and most notably, an important character trait to foster in youth: grit. Defined as passion and perseverance for the achievement of long-term goals, (Duckworth, Peterson, Matthews, & Kelly, 2007), grit has been identified by the United States Department of Education as a critical factor for success in the 21st century (Shechtman, DeBarger, Dornsife, Rosier, & Yarnall, 2013) as it is found to be directly related to students’ GPA and inversely related to hours spent watching television (Duckworth & Quinn, 2009). Measures for positive youth development (Bowers et al., 2010) and grit (Duckworth et al., 2007) will be explained. Second, this paper will present an on-going study collecting prospective, longitudinal data on positive youth development and grit for Friend Fitness participants. Preliminary findings from this research indicate that participants experienced an overall increase in grit, and that this character trait was related to positive youth development. These are groundbreaking findings that necessitate continued investigation due to their implications on physical, cognitive, social, and behavioral development in adolescence. It is hoped that this study will garner interest for future research on grit interventions and school-based positive psychology initiatives with the goal of supporting students to have higher well-being, school engagement, positive relationships, achievement, and fewer physical health problems such as obesity.