A growing literature reports significant associations between children's executive functioning skills and their mathematics achievement. The purpose of this study was to examine if specific early number skills, such as quantity discrimination, number line estimation, number sets identification, fast counting, and number word comprehension, mediate this association. In 141 kindergarteners, cross-sectional analyses controlling for IQ revealed that number sets identification (but not the other early number skills) mediated the association between executive functioning skills and mathematics achievement. A longitudinal analysis showed that higher executive functioning skills predicted higher number sets identification in kindergarten, which in turn predicted growth in mathematics achievement from kindergarten to second grade. Results suggest that executive functioning skills may help children quickly and accurately identify number sets as wholes instead of getting distracted by the individual components of the sets, and this focus on sets, in turn, may help children learn more advanced mathematics concepts in the early elementary grades. (author abstract)