This longitudinal study investigated the roles of elementary schoolers’ executive function (EF) and theory of mind (ToM) as predictors in their academic achievements in Japanese language skills (e.g., speaking and listening ability, writing ability, reading ability, and linguistic knowledge) from first through fifth grade. We assessed the EF, ToM, and academic achievements of 85 Japanese elementary schoolers, when they were grade 1 ( M = 90.19 months) and grade 3 ( M = 114.34 months). Moreover, academic achievements of same children tested in grade 5 ( M = 137.92 months). A path analysis revealed that EF abilities in Grades 1 and 3 predicted Japanese language achievement via ToM ability, and EF and ToM in Grade 1 had an indirect effect on achievements in Grades 3 and 5. Further, Japanese language skills achievement in Grade 1 predicted EF ability in Grade 3 and EF predicted achievement in Grade 5. These findings indicate the vital role of elementary schoolers’ EF on academic achievement in Japan.