There is evidence that mastery motivation contributes to developmental outcomes, both for typically developing individuals and for those with disabilities such as Down syndrome. Mastery motivation appears to be a stable trait, at least during early childhood, but research with adults has been restricted by the absence of an appropriate measure. The current study investigated the stability and consistency of parent-reported mastery motivation for 21 individuals with Down syndrome across 20 years. Parents completed the Dimensions of Mastery Motivation Questionnaire (DMQ) when their child was aged 4-6 years and 11-15 years. They completed the Dimensions of Adult Mastery Motivation Questionnaire (DAMMQ) when their son or daughter was 23-27 years old. There were strong positive correlations among measures of mastery motivation across the three time points. Persistence decreased significantly from early childhood to adolescence but increased from adolescence to adulthood. Most individual profiles fitted this pattern, with only one of the 21 participants showing continual declines in persistence. For individuals with Down syndrome, patterns of mastery motivation may be developed early in life and sustained into adulthood.