Executive function impairment is highly prevalent and debilitating in persons with multiple sclerosis (MS). Recent research efforts have sought to better understand the neural underpinnings of executive dysfunction in this population using functional neuroimaging paradigms examining the prefrontal cortex (PFC). The present cross-sectional study examined PFC activation during Trail-Making Test (TMT) performance in persons with MS and healthy controls using a functional near-infrared spectroscopy (fNIRS) approach. Ten adults with MS and 10 healthy control participants were fitted with an fNIRS system and underwent the TMT (i.e., both TMT-A and TMT-B conditions) during a single laboratory visit. There were no statistically significant differences between groups in behavioral performance on the TMT (i.e., non-significant group by condition interaction (p = 0.19) on TMT time to completion). Further, there was a non-statistically significant group by condition interaction for HbO₂ levels during the TMT (p = 0.65). However, there was a significant main effect of TMT condition where PFC HbO₂ activation levels increased during TMT-B compared to TMT-A (p < 0.01). There was a statistically significant main effect of group on HbO₂ levels, such that lower HbO₂ levels were observed in MS across TMT conditions compared with controls (p < 0.01). Greater HbO₂ levels were strongly and significantly correlated with worse TMT-B performance in the MS sample (r = 0.66, p = 0.04), but not in the healthy control sample (r = 0.17, p = 0.63). Overall, despite demonstrating similar behavioral performance as healthy controls on the TMT, persons with MS had substantially less PFC HbO₂ activation for both TMT-A and TMT-B conditions; this seemingly did not depend on task difficulty. We speculate that this generalized reduced PFC activation in MS might reflect disruptions in neural networks that are important for executive functioning that may not have been captured by fNIRS.