This study aims to test the theory underlying Job Corps, one of the largest education and training programs in the U.S. serving disadvantaged youth. Central to the program are vocational training and general education that serve as two concurrent mediators transmitting the program impact on earnings. To distinguish the relative contribution of each, we develop methods for decomposing the Job Corps impact on earnings into an indirect effect transmitted through vocational training, an indirect effect transmitted through general education, and a direct effect attributable to supplementary services. We further ask whether general education and vocational training reinforce each other and produce a joint impact greater than the sum of the two separate pathways. Moreover, we examine the heterogeneity of each causal effect across all the Job Corps centers. This article presents concepts and methods for defining, identifying, and estimating not only the population averages but also the between‐site variance of these causal effects. Our analytic procedure incorporates a series of weighting strategies to enhance the internal and external validity of the results and assesses the sensitivity to potential violations of the identification assumptions.