Functional near-infrared spectroscopy (fNIRS) has emerged as a promising technique in motor cognitive neuroscience and has become an important neuroimaging tool for the study of motor cognition. This review synthesizes evidence from fNIRS studies to elucidate the neural mechanisms that underlie exercise-induced improvements in executive function in older adults. A systematic search was conducted across six electronic databases from inception to March 20, 2025, and 27 relevant articles were included. These studies were systematically reviewed to examine the neural mechanisms by which exercise improves executive function in older adults along five dimensions: (1) resting-state brain activity; (2) task-evoked brain activity during executive function tasks; (3) acute exercise-induced immediate improvement in brain activity; (4) sustained effects on brain activity following acute exercise; and (5) long-term enhancements in brain activity after regular physical exercise. The results showed that a decrease in cerebral oxygenation accompanied brain aging, weakened hemodynamic oscillations, and abnormal resting-state functional coupling. A two-stage neural compensation model may underlie the exercise intervention aimed at improving executive function in older adults. Acute exercise can temporarily improve executive function by expanding the “resource pool” to increase neural resources and enhance prefrontal cortical hemodynamic activity and recruitment of neural resources. Chronic exercise achieves structural–functional optimization and efficient use of neural resources through the accumulation effect of repeated acute exercise stimulation, thereby continuously improving executive function. Therefore, we suggest that future studies should conduct large-scale RCTs using multimodal neuroimaging methods combining ERP, fMRI, and fNIRS. This will compensate for the shortcomings of fNIRS and provide a deeper understanding of how exercise remodels brain networks, thereby establishing a theoretical basis for precision interventions targeting brain aging.