The purpose of this mixed methods study was to examine the sources of food and dietary patterns of Tongan adolescents (n = 2084) and their perceptions of sociocultural influences. The study incorporated anthropometric measurements, a behavioral survey, and qualitative interviews. More adolescent Tongan females (82.5%) than males (74.3%) reported sourcing morning tea and/or lunch (females 81.9%, males 72.6%) from school canteens or nearby food outlets. More females than males reported consuming obesity-promoting foods such as packaged snack foods (females 38.2%, males 21.3%), chocolates (females 24.7%, males 15.0%), and soft drinks (females 55.3%, males 50.4%). Food purchased for consumption at schools was predominantly energy dense and nutrient poor. Ensuring that students have access to foods of high nutritional quality sourced from school or home, and restricting access to local food outlets that supply unhealthy products would improve the nutrition status of adolescents in Tonga. Furthermore, it is important that obesity prevention interventions are informed by culture-specific influences to optimize uptake of healthy diets.