Knowledge of mechanisms linking early-life social environment and breast cancer remains limited. The authors explore direct and indirect effects of early-life socioeconomic status (SES) on breast cancer prevalence in later life. Using 50year data from the Wisconsin Longitudinal Study (N = 4,275) and structural equation modeling, the authors found a negative direct effect of early-life SES, indicating that women from higher SES family backgrounds had lower breast cancer prevalence than women from lower SES families. Additionally, early-life SES has a positive indirect effect on breast cancer via women’s adult SES and age at first birth. Were it not for their higher SES in adulthood and delayed childbearing, women from higher SES families of origin would have had lower breast cancer prevalence than women from lower SES families. Yet early-life SES is associated positively with adult SES and age at first birth, and women’s higher adult SES and delayed childbearing are related to higher breast cancer prevalence.