The purpose of this study was to determine the neural and muscular correlates of physical and mental fatigue. Twelve young and healthy participants (balanced by gender) performed three fatiguing conditions: 1) isometric handgrip task (HG) at 30% maximum voluntary contraction (MVC); 2) mental arithmetic (MA); and 3) combined handgrip and mental arithmetic (HG+MA). Prefrontal cortex activation, muscle activity, and task performance measures (force fluctuations and mental performance) were obtained continuously during the fatiguing conditions. Results indicated no significant difference in muscle activity and force fluctuations between the HG and HG+MA condition. Compared to the HG condition, an initial increase in the prefrontal cortex activation was observed during the HG+MA condition, with a subsequent decrease at exhaustion. Cortical activation during the MA condition was significantly lower than the other conditions across the entire duration of the fatiguing trial. Participants performed poorly with mental arithmetic during the HG+MA condition compared to the MA condition. The results suggest that the dynamic changes in the prefrontal cortex during the combined HG+MA condition, compared to the HG or MA condition, occur due to the interaction of physical and mental fatigue, which may have important implications for physical and cognitive performance.