Objective: Individuals with ADHD are often characterized as inconsistent across many contexts. ADHD is also associated with deficits in executive function. We examined the relationships between response time (RT) variability on five brief computer tasks to parents’ ratings of ADHD-related features and executive function in a group of children with a broad range of ADHD symptoms from none to full diagnosis. Methods: We tested 98 children (mean age 9.9 ± 1.4 years; 66 boys) from community clinics on short tasks of executive control (TEC) and the Eriksen Flanker task, while a parent completed the Conners’ Parent Rating Scale and Behavior Rating Inventory of Executive Function. Results: Variability for two of the TEC tasks explained significant proportions of the variance of all five ADHD-related Conners’ subscales and several executive function subscales. By contrast, variability on the flanker task or mean RTs for any task were not associated with any rating scale. Conclusion: The significant dimensional relationships observed between variability measures and parent ratings supported the utility of RT variability as an objective measure in ADHD and aspects of executive functioning that is superior to RT means or accuracy measures.