Objective: Reaction time (RT) variability is often purported to indicate behavioral attention. This study seeks to examine whether RT variability in children with Attention Deficit Hyperactivity Disorder (ADHD) is associated with observed behavioral indicators of attention. Method: One-hundred 47 participants with and without ADHD completed five computerized neuropsychological tasks and an analog math task. Linear mixed models were utilized to examine the relationship between observations of behavioral inattention during the analog task and measures of RT variability from the neuropsychological tasks. Results: Significant associations were observed between RT variability and mean duration of on-task behavior on the analog math task. Secondary analyses indicated that on-task behavior during the math task was also related to accuracy on the neuropsychological tasks. Conclusions: RT variability, especially the portion of RT variability characterized by long RTs, appears to measure a cognitive phenomenon that relates to successful on-task academic behavior across children with and without ADHD. The relationship between RT variability and on-task behavior is present across multiple neuropsychological tasks and does not appear to be moderated by age, sex, or the presence of anxiety or depression.