Introduction: The purpose of the current study was to analyze the cross-sectional developmental trajectories of explicit category learning in individuals with Down syndrome compared to individuals with intellectual disability and typically developing individuals. Explicit learning is active, conscious, controlled, and intentional; it is a deliberate attempt to acquire new knowledge or skill from repeated tries with feedback. Explicit learning improves with age throughout childhood and is closely related to intelligence. Because of its relation to intelligence, we expected individuals with Down syndrome to perform below the level expected for their chronological age and nonverbal ability. Methods: The sample was comprised of 41 individuals with Down syndrome, 25 individuals with intellectual disability, 40 individuals who were typically developing chronological age matches, and 27 individuals who were typically developing nonverbal mental age matches. All participants completed a measure of nonverbal ability, the Leiter International Performance Test-Revised, and two measures of explicit learning, the Category Task and the Concept Formation subtest of the Woodcock-Johnson-III. Results: Cross-sectional developmental trajectories were created examining explicit learning over chronological age and explicit learning over nonverbal ability. For the Category Task over chronological age trajectory, the Down syndrome and intellectual disability groups had a delay in onset in explicit learning. For the Woodcock-Johnson-III over chronological age trajectory, the Down syndrome and intellectual disability groups had a delay in onset in explicit learning, and the Down syndrome group showed a slower rate in development in explicit learning. For the Category Task over nonverbal ability trajectory, no group showed a delay in onset or slower rate in development in explicit learning. For the Woodcock-Johnson-III over nonverbal ability trajectory, the Down syndrome group had a slower rate of development in explicit learning. Conclusion: The results suggested that in comparison to typically developing individuals and individuals with mixed-etiology intellectual disability, individuals with Down syndrome show similar performance in and development of explicit category learning in relation to their nonverbal ability as long as the explicit learning measure does not constrain their performance.