The role of early childhood mastery motivation as a predictor of executive function 20 years later was examined in a sample of 39 individuals who had early diagnosed developmental disabilities. Multilevel modeling was used to analyze predictors of accuracy and response time on a Flanker task measuring executive function. As predicted, participants had relatively poorer performance on trials requiring inhibition and rule switches. Individuals with Down syndrome, in comparison to other participants, demonstrated longer response times. Young adults who had higher levels of persistence on mastery motivation tasks during early childhood displayed higher levels of accuracy and shorter response times on the executive function task. Possible mechanisms by which early mastery motivation relates to later executive function are discussed.