This study was performed to investigate connections between attention deficit hyperactivity and early school adjustment for first–grade children. In order to expand the scope of understanding on the issue, research was expanded to include the possible mediating effects of executive function impairment. The subjects consisted of 1,598 elementary school students, whose data were taken from the Panel Study of Korean Children (PSKC) 8th wave beginning in 2008 collected by KICCE(Korea Institute of Child Care and Education). The major findings of this study were as follows. School adjustment was negatively correlated with attention deficit hyperactivity and executive function impairment in first grade children. Children with a higher score in ADHD symptoms had significantly lower school adjustment scores as well as higher executive function impairment scores. Children’s executive function impairment was also found to be a significant mediator in the relationship between attention deficit hyperactivity and early school adjustment in first–grade children. These findings indicate that executive function impairment may function as a pathway that makes it difficult for first graders with attention deficit hyperactivity to adjust to school life.