Cognitive functions, including memory, perception, learning, attention, language abilities, problem solving, and decision making, are impaired in a range of neurodegenerative, neurodevelopmental, and neuropsychiatric diseases. Current therapeutic options are limited to the pharmacological and behavioral therapies. Neuromodulation therapies that target specific neural activities and brain areas require robust electrophysiological biomarkers that indicate cognitive performance in clinically validated tests. To identify specific cortical areas and spectral activities that indicate behavioral differences between good and poor performers we used EEG recordings from 86 epilepsy patients during performance of a CANTAB battery of tasks with standardized behavioral measures of visual memory, spatial memory, working memory, and executive functions. EEG electrode signals during performance of particular battery tasks were decomposed to identify specific frequency bands and cortical areas that differentiated patients with impaired and preserved functions normalized by their age and gender. The anterior prefrontal cortical EEG power in the theta frequency band was consistently lower in patients with impaired cognitive performance (z-score < -1). This effect was evident in all four behavioral measures and was confined to the cortical area of the four frontal pole electrodes (Nz, Fpz, Fp1, Fp2). Theta EEG power in the anterior prefrontal cortex provides a simple, accessible, and objective electrophysiological measure of memory and executive functions. Our results suggest a feasible clinical biomarker for diagnosis, monitoring, and treatment of cognitive deficits with new neuromodulation therapeutic approaches in epilepsy and potentially also other brain disorders.