The presence of Mild Cognitive Impairments (MCI) is associated with worse gait performance. However, the effect of MCI on cortical control of gait, as assessed during active walking, is unknown. We hypothesized that MCI would be associated with attenuated activations and limited improvement in efficiency in the Prefrontal cortex (PFC) under cognitively-demanding walking conditions. Functional Near-Infrared Spectroscopy (fNIRS) was used to assess Oxygenated Hemoglobin (HbO2) in the PFC during Single-Task-Walk (STW), cognitive interference (Alpha) and Dual-Task-Walk (DTW) conditions. Three repeated trials in each experimental condition were administered. Healthy control (n = 71; mean age = 76.82 ± 6.21 years; %female = 50.7) and MCI (n = 11; mean age = 78.27 ± 4.31 years; %female = 45.5) participants were included. The increase in HbO2 from STW to DTW was attenuated among MCI participants compared to controls (estimate = 0.505; p = 0.001). Whereas, among controls, HbO2 increased from Alpha to DTW, the opposite was observed among MCI participants (estimate = 0.903; p < 0.001). In DTW, the decline in HbO2 from trial 1 to 2 was attenuated in MCI participants compared to controls (estimate = 0.397; p = 0.008). Moreover, whereas HbO2 declined from trial 1 to 3 among controls, MCI participants showed the opposite trend (estimate = 0.946; p < 0.001). MCI was associated with attenuated brain activation patterns and compromised ability to improve PFC efficiency during dual-task walking.