This paper examines the relationship between the developmental contexts of youth programs and resultant developmental outcomes, and explores whether the developmental process of thriving mediates this relationship. Developmental context is proposed to consist of three elements: (1) youth sparks, (2) program quality, and (3) developmental relationships. Combined, these elements describe youth program context more precisely than in previous studies, allowing for a clearer understanding of effective program settings. Likewise, the process of youth thriving provides insight into the mechanism through which youth development occurs. Sufficient model fit, convergent validity, and discriminant validity of the 4-H Thriving scale were determined through a multi-phase confirmatory factor analysis. As hypothesized, structural equation modeling revealed a full mediational effect of youth thriving on developmental outcomes. The results of this study guide youth development practitioners to focus on the quality of the developmental context of youth programs and the ways in which programs can promote youth thriving.