We examine a potential mechanism underlying the relation between executive function (EF) skills and mathematics achievement in early childhood. Across two samples of three- to six-year-olds in preschool and kindergarten, we found that children’s EF skills predict their concurrent skills in set counting, numeral identification, number comparison, and number line estimation. The effects of EF on later numeral identification, number comparison, and number line estimation skills remained significant for these two samples of children, but these effects attenuated when controlling for the respective earlier numeracy skill. Further, aspects of numeracy skills mediated the association between EF and mathematics achievement in both samples. Together, these findings provide evidence on the nuanced relations between EF, numeracy, and mathematics achievement, and suggest attention to each numeracy skill in order to support early mathematical development. (author abstract)