The purpose of this study was to examine the effect of supported employment intervention on the employment outcomes of transition-age youth with intellectual and developmental disabilities served by the public vocational rehabilitation system using a case-control study design. Data for this study were extracted from the Rehabilitation Services Administration Case Service Report (RSA-911) database for fiscal year 2009. The sample included 23,298 youth with intellectual and developmental disabilities aged between 16 and 25 years old at the time of application. The classification and regression tree (CART) method was used to estimate propensity scores and to adjust for selection bias on the basis of all prominent covariates relevant to the dependent variable (i.e., competitive employment). Results yielded six homogeneous subgroups, and receipt of supported employment was found to increase the employment rates across all of the groups. The effect of supported employment was especially strong for youth who were Social Security beneficiaries, special education students, and individuals with intellectual disabilities or autism who were high school graduates. These findings suggest that supported employment is an effective service for enhancing the vocational rehabilitation outcomes of young adults and provides valuable information for policy makers, health care providers, rehabilitation counselors, and educators.