Soy isoflavones are weak estrogenic compounds contained in products derived from soybeans, including soy-based infant formula. Exposure to these compounds in infancy may have lasting effects on later neurological and reproductive development and function. This study examined the association between early life soy-based feeding and developmental outcomes, including language acquisition, gender-role play behavior and time-to-menarche. Subjects were participants in the Avon Longitudinal Study of Parents and Children (ALSPAC). Subjects were classified into mutually exclusive infant feeding categories: early soy, late soy, primarily breast, and early formula (referent). Language acquisition, measured as word comprehension and production, was assessed using the MacArthur Communicative Development Inventory (MCDI) at 15 and 24 months of age. Gender-role play behavior was assessed using the Preschool Activities Inventory (PSAI) at 42 months of age. Time-to-menarche was assessed by self-report of age at menarche between ages 8 and 14. Using generalized estimating equations (n = 3,384 boys; 3,176 girls), a small, imprecise increase in both word comprehension and word production was observed in girls with early soy exposure (βcomprehension = 0.87, 95% CI: 0.36, 1.38; βproduction = 1.46, 95% CI: 0.56, 2.29) over time, as compared to the referent. No association was observed among boys. Likewise, no association between PSAI score and infant feeding method was observed in boys, using multivariable linear regression models. However, the mean PSAI score among early soy exposed girls was slightly higher than the referent (β = 2.68, 95% CI: 0.20, 5.15), indicating slightly masculinized behavior. Time-to-menarche was assessed in 2,884 girls using Cox proportional hazards modeling. The rate of menarche in early adolescence (before age 12.5) increased by 42% in the early soy fed girls (Hazard Ratio: 1.42, 95% CI: 0.92, 2.20). There was no association in later adolescence. In this study, early life exposure to soy products was associated with slight, imprecise associations with developmental outcomes in girls, but not boys. Interpretation was limited by a small number of early soy exposed subjects. Despite the imprecision, similar patterns of association among girls in all outcomes support the need for additional studies to replicate these novel findings.