The purpose of this study was to investigate the relationship between kindergartners’ school readiness (SR) and their academic performance (AP) in first grade, as well as the mediating effect of executive function (EF), if any. Data from the 7th (2014, kindergartners) and 8th (2015, first graders) Panel Study on Korean Children (PSKC) were used in this study. Results showed that children’s AP was significantly related to their EF and SR. Furthermore, low scores for SR was associated with more EF difficulties in children. Children’s EF and SR predicted their AP at significant levels. Moreover, all path coefficients were statistically significant. Children’s SR had a direct influence on their AP a year after they joined school, and an indirect influence on EF. Both the total effect from SR to AP, and the indirect effect were significant. In conclusion, the EF played a partial mediating role on a child’s SR to AP.