The goal of this study was to explore the long-term effects of all-day kindergarten programs on children's academic performance. The study used three waves of data from a nationally representative database from the United States, the Early Childhood Longitudinal Study (ECLS), with the first wave at the beginning of kindergarten and the third wave at the end of the first grade. The average age of the children at the first wave was five years and eight months; at the third wave seven years and three months. The study further examined if these effects vary as a result of teacher curricular activities relating to reading and mathematics. The study conducted three-level longitudinal multilevel analyses using three measures of reading and maths IRT scores, sets of teacher activities in reading and maths, children's age, gender and socioeconomic status. The results indicated that all-day kindergarten children began with significantly higher scores and showed faster growth rates in both reading and math compared with half-day kindergarten children. Importantly, all-day kindergarten teachers had significantly higher frequencies of reading and mathematics activities than did half-day kindergarten teachers. These results suggest an important implication for the potential benefits of all-day programs in the US.