Deficits in executive function have been associated with internalizing problems in children. Yet little is known about the mechanisms that may explain this association. Using longitudinal data across elementary school years (N = 1,364), this study examined the role of peer difficulty and poor academic performance in understanding longitudinal associations between executive function and internalizing problems. Executive function was measured in first grade with observed tasks and standardized tests. Peer difficulty and academic performance were reported by teachers and/or mothers at three waves. Internalizing problems were reported by mothers at four waves. Using structural equation modeling, results demonstrated that peer difficulty and poor academic performance independently mediated longitudinal relations between executive function and internalizing problems. Findings highlighted the importance of children's functioning in key identity domains in understanding the adverse impact of inferior executive function on internalizing problems in school-age children.