Extracurricular activity participation (EAP) has been positively linked with increased academic achievement and college attendance. However, the mechanisms linking EAP to educational outcomes are poorly understood. Using the Education Longitudinal Study of 2002 (ELS), this study contributes to our understanding of the relationship between EAP and educational success by (1) examining the ability of nine educational, social, and developmental mechanisms to explain the link between EAP and high school math achievement gains and the chances of 4-year college attendance and (2) examining the ability of family income to moderate the influence of these mechanisms. Results suggest that educational expectations, noncognitive skills, and social capital in the form of communication among parental groups are meaningful mediators, regardless of family income. These findings extend the literature concerned with understanding how EAP is related to academic outcomes, a connection that is not necessarily intuitive.