Background: While the effects of early sleep problems on children's development may persist for some time, there is limited understanding in the field regarding whether these effects persist into adolescence. Research is needed to address this issue, as it is an important area to investigate to assess its influence on children's success in school and in life. Objective: The goal of the study was to examine the extent to which children's sleep problems in preschool years can predict later cognitive and socioemotional functioning from middle childhood to adolescence. Method: By using the four-wave NICHD Early Childcare Research Network dataset (N = 1364), this study considered direct and indirect associations between variables by employing structural equation modeling with the bootstrapping method and an extended cross-lagged mediation model. Results: Sleep problems at age 3 were directly associated with socioemotional difficulties in grade 1 and at age 15, and were indirectly associated with socioemotional difficulties in grade 3. However, no significant effect of early sleep problems on later cognitive functioning was observed. Conclusions: Preschool-aged children's sleep problems contribute to an increased risk of socioemotional problems rather than impairment of cognitive functioning in middle childhood and adolescence, and that unresolved sleep problems during preschool years can be considered a risk factor for poor socioemotional development.