The current study investigated the contribution of 5 hypotheses to the estimated effects of preschool in the Child-Parent Centers on occupational prestige, felony arrest, and depressive symptoms in adulthood in the Chicago Longitudinal Study. An alternative-intervention, quasi-experimental design included over 1,400 low-income participants (93% of whom were Black) who attended preschool for 1–2 years or the usual early educational intervention and were traced to age 24. LISREL analysis of 5 hypotheses (cognitive advantage, family support, school support, motivational advantage, and social adjustment) indicated that while each individually accounted for part of the estimated direct effect of preschool on adult well-being, the best fitting model across outcomes included indicators of all 5 hypotheses. The full model completely accounted for the direct effect of preschool on occupational prestige and official felony arrest, and 79% on depression symptoms. Key mediators included cognitive skills at school entry, school quality in the elementary grades, juvenile arrest, and school completion. The identified processes may help establish, strengthen, and sustain effects in other programs and settings.