Background: Mirror therapy is widely used in stroke rehabilitation, yet its immediate task-related cortical responses remain incompletely characterized. We used functional near-infrared spectroscopy (fNIRS) to characterize cortical hemodynamic responses during a mirror task in patients with stroke. Methods: Fifty-one stroke patients with upper limb dysfunction were recruited. Participants performed both a mirror task and a control task, while hemodynamic changes in regions of interest were simultaneously recorded using a 43-channel fNIRS device. General linear model (GLM) analysis was used to compare differences in activation intensity between the mirror task and control task in the cortical regions of interest. Results: Compared with the control task, the mirror task significantly activated 10 channels: CH2, CH3, CH5, CH9, CH11, CH19, CH24, CH25, CH32, and CH33. The activated brain regions were concentrated in the bilateral premotor cortex/supplementary motor area (PMC/SMA), ipsilesional supramarginal gyrus (SMG), ipsilesional primary somatosensory cortex (S1), and contralesional primary motor cortex (M1). Conclusions: The mirror task elicited greater activation than the control task across a distributed cortical network involving bilateral PMC/SMA, ipsilesional SMG/S1, and contralesional M1 in patients with stroke. These findings demonstrate that fNIRS can objectively characterize task-related cortical hemodynamic responses during mirror tasks and provide a basis for future studies linking these cortical response patterns to rehabilitation outcomes.