Physical activity (PA) is increasingly recognized as an effective approach for improving cognitive function. However, it is unclear how short-term PA or physical inactivity might influence cognition, and whether this effect is associated with other health outcomes such as vascular function. A series of three studies were conducted to address this knowledge gap. The first study systematically reviewed and meta-analyzed 90 effects from 16 randomized controlled trials to examine the effect of acute exercise on cognitive function in healthy older adults aged 60 years or above. The results revealed that acute exercise elicited a small but significant improvement in cognitive function compared to the non-exercise control condition (standardized mean difference [SMD] = 0.17, p = 0.003), and greater improvements were found in time- than accuracy-related cognitive outcomes (SMD = 0.24 versus 0.11, p < 0.05). The second study investigated the effects of a single bout of moderate-intensity walking on cognitive and vascular function in physically inactive older adults aged 60 years or above. Compared to the control (sitting) condition, acute exercise (30-min walking at 100 steps/min) significantly improved performance in executive function, whereas reduced carotid to femoral pulse wave velocity (p-values < 0.05). Changes in processing speed and aortic augmentation index were similar between conditions (p > 0.05), whereas central mean arterial blood pressure was increased after sitting (p < 0.05) but remained unaltered after acute exercise (p > 0.05). The third study focused on the impact of short-term physical inactivity (i.e., walking below 5000 steps/day for one week) on cognitive performance and vascular function among physically active individuals aged 50 years or above. Results from the Bayesian analysis demonstrated minimal changes in all variables before versus after step reduction, indicating that one week of reduction in PA did not yield detrimental effects on cognitive performance and vascular function. Collectively, this body of work supports a small but positive effect of acute exercise on cognition among generally healthy older adults without cognitive impairment. In addition, significant associations were found between changes in cognitive performance with arterial stiffness and central blood pressure in response to acute exercise or short-term PA reduction.