The prefrontal cortex (PFC) not only plays a less mathematics-specific role in number processing and calculation but it is also responsible for working memory demands coping. Through functional near-infrared spectroscopy imaging, this study explored how differently the PFC of 31 subjects with contrasting mathematical fluency would respond to increasing working memory demands of mental arithmetic task. Area under the curve analyses of the oxygenated hemoglobin signals, hereinafter representing brain activation, were compared across levels and groups. Depending on task performance, the subjects were allocated to either normal performers (NP) or high performers (HP) group. Both groups showed sensitivity with regards to task performance (i.e., the number of problems attempted and accuracy) towards increasing working memory demands (level 1 to 3). With increasing task level, NP's PFC activation increased while HP's remained unchanged. NP also showed consistent greater PFC activation than HP in all three levels. These findings implied that the PFC plays a more auxiliary role to cope with working memory demands imposed by mental arithmetic on individuals who are less fluent in mathematics. This can potentially serve as a framework to monitor workload hemodynamics continuously and unobtrusively. © 2020 IEEE.