Sluggish Cognitive Tempo (SCT) refers to a clinical construct including several symptoms such as sluggishness, absentmindedness, low energy. In the present study, we compared neurocognitive laboratory outcomes of ADHD children with or without SCT. Method: The CNS Vital Signs Battery was utilized to measure neurocognitive measure of the participants. The SCT + ADHD group comprised of 42 subjects, ADHD group was 41 subjects and control group was 24 subjects. Results: The cognitive flexibility score was found to be more severely impaired in ADHD children with SCT in comparison to the ADHD-only. Additionally, greater deficits in the Shifting Attention Test (p = 0.014) and the Continuous Performance Test (reaction time score, p < 0.01) were found in the SCT + ADHD group relative to ADHD group. Processing speed, visual/auditory memory, psychomotor speed and reaction time were not found to more impaired in those comorbid with SCT. Conclusion: Impairments in the cognitive flexibility and more specifically shifting attention and continuous performance may be indicative of vigilance and orientation problems rather than executive functions for the SCT construct.