Understanding how domain-general and domain-specific cognitive characteristics impact school readiness is crucial for developing improved early interventions for preschoolers living in poverty. This longitudinal study investigated the structure and development of domain-general executive functioning skills and the development of domain-specific mathematics skills across the preschool year in children attending Head Start. Using confirmatory factor analyses, results indicated that a unidimensional factor structure best fit executive functioning data at both the fall and spring time points, and this structure was stable over time as indicated by tests of measurement invariance. After controlling for both age and verbal ability, structural equation modeling analyses revealed that executive functioning skills in the fall predicted mathematics skills in the spring, and mathematics skills in the fall were also predictive of children's spring executive functioning scores. These relationships did not hold after controlling for fall scores on executive functioning and mathematics. To interpret the finding that fall executive functioning and fall mathematics were highly correlated, theoretical models of cognitive dissociation and specialization are discussed. Implications of these findings for early intervention and future research directions are highlighted.