The aim of this study was to reanalyze several datasets to investigate the relation of childhood aerobic fitness on cognition via a number of task performance outcomes and the P3 event-related brain potential component during a modified flanker task, which modulates inhibitory control demands. In a sample of 702 preadolescent children (ages 8–11; 342 females), demographic measures of age, sex, IQ, socioeconomic status, and pubertal status were considered along with aerobic fitness (i.e., VO₂peak%). Correlational analyses were conducted to determine the influence of the demographic variables and aerobic fitness on inhibitory control outcomes of response accuracy, reaction time (RT), response variability, interference control, cognitive strategies, P3 amplitude, and P3 latency. Subsequent hierarchical regression analyses were performed with significant demographic factors in the first step and aerobic fitness in the final step. Results indicated that after accounting for significant demographic variables specific to each dependent outcome, aerobic fitness was positively related to response accuracy, with no such relations observed for response speed (RT) for congruent and incongruent conditions of the flanker task. However, greater aerobic fitness was associated with less RT variability and lower accuracy interference. In terms of cognitive strategies, greater fitness was also associated with better discrimination accuracy as well as faster and better quality of information uptake. Across a 15-site region of interest around the topographic maximum, findings indicated that aerobic fitness was positively associated with larger P3 amplitude during incongruent trials. No such relation was observed for P3 latency. Relying on a large aggregated dataset, we demonstrated that aerobic fitness may be particularly beneficial to the allocation of attentional resources, as indexed by P3 amplitude as well as response accuracy and intraindividual variability. These findings, while generalized across inhibitory control demands, were especially related to trials that required greater amounts of inhibition. Thus, aerobic fitness may benefit brain and behavioral outcomes during childhood and has public health implications for the role of childhood physical activity on aspects of cognition that underlie scholastic performance and lifelong effective functioning.