Background Functional near-infrared spectroscopy (fNIRS) is widely used to measure cognitive load during working memory tasks. However, little is known about how task structure modulates load-dependent neural responses. Objective We examined how working memory load measured by a verbal memory task is represented in fronto-parietal hemodynamic responses during the encoding phase in cognitively healthy older adults and whether these responses are modulated by the predictability of task difficulty. Methods Oxygenated hemoglobin (HbO) responses were measured using fNIRS while participants performed digit span trials with load levels presented in either increasing order or randomized sequences. Results Mixed-effects modeling across all participants and fronto-parietal regions of interest revealed a significant main effect of load levels, with HbO responses during the encoding phase increasing linearly with increasing memory load. However, load sensitivity differed by task structure: the random group showed a significantly steeper load-related increase in HbO responses than the non-random group, driven primarily by the left frontal cortex. Behavioral accuracy remained stable across load levels. Conclusions These findings indicate that fronto-parietal encoding-phase hemodynamic responses reflect working memory demands in a context-dependent manner, revealing neural sensitivity beyond overt behavioral performance.