Objective: Previous studies of cognitive decline in patients with neurocognitive disorder due to traumatic brain injury (NCD-TBI) have often failed to control for baseline factors such as premorbid intelligence. The purpose of the current study was to estimate and compare cognitive function among three groups (controls, complicated mild/moderate TBI, and severe TBI) after controlling for premorbid intelligence. Methods: Severity of TBI was classified as complicated mild/moderate or severe based on duration of loss of consciousness and brain neuroimaging results. Premorbid intelligence quotients (IQs) were estimated with the Oklahoma Premorbid Intelligence Estimate. There were no differences in premorbid intelligence between the groups, which were also matched for age and education. Current cognitive function was evaluated with the Wechsler Adult Intelligence Scale-Fourth Edition. Results: Comparison of current cognitive function among the three groups indicated significant group differences for all indexes and subtest scores. Processing speed showed the highest effect size. However, only working memory differed significantly between the two NCD-TBI groups. Conclusion: The present findings suggest that mental memory manipulation processes seem to be more sensitive to TBI severity than are perceptual-motor processes. Specifically, both auditory rehearsal/discrimination and mental alertness/manipulation will be most strongly influenced by TBI severity.