Cognitive subtyping may be essential for understanding shared mechanisms underlying attention-deficit/hyperactivity disorder (ADHD) and autism spectrum disorder (ASD). Participants from a population-based sample ( n = 360) and an ASD/ADHD clinic-based sample ( n = 254), ages 5 to 17 years, were administered a broad cognitive battery. Latent class analyses revealed a similar four-cognitive-class solution in both samples mainly characterized by speed–accuracy tradeoff differences instead of specific strengths and weaknesses. The classes were strongly predictive of both ASD and ADHD (and comorbid) symptoms in the clinical, but not the population, sample. Results support the hypothesis that both disorders are manifestations of the same overarching disorder and illustrate the effectiveness of subtyping based on cognitive profiles. Results also support a step-function endophenotype model, in which cognitive problems mediate the gene–psychopathology associations but only in a clinically relevant way in the subgroup of children with high genetic susceptibility. Pre-onset longitudinal research is needed to corroborate this.