The interrelationships between math and behavioral skill development prior to school entry are not well understood, yet have important implications for understanding how to best prepare young children for kindergarten. This study addresses this gap by utilizing a sample of 1,750 children (53% male; 47% White, 16% Black, 16% Hispanic, 8% Asian, and 13% other or multirace) from the Early Childhood Longitudinal Study-Birth Cohort (ECLS-B) to examine the interrelationships between behavioral (parent-reported prosocial skills and externalizing problems) and math skills (directly assessed) across the ages of 4, 5, and 6 years. Results from a random intercept cross-lagged panel model (RICLPM) revealed significant between-person associations between each of these skills over time. After controlling for these between-person differences, findings identified several significant within-person associations both between and within domains, with prosocial skills at age 4 predicting both prosocial and math skills at age 5. At age 5, math skills were predictive of future math and prosocial skills at age 6. Externalizing problems were not significantly associated with either of the other skills at any time points, but did predict continued externalizing problems from ages 5 to 6. Findings highlight the importance of supporting children's prosocial and math skills in conjunction with one another during early childhood prior to school entry.