Vitamin D, a multipurpose steroid hormone vital to health, has been increasingly implicated in the pathology of cognition and mental illness. Hypovitaminosis D is prevalent among older adults, and several studies suggest an association between hypovitaminosis D and basic and executive cognitive functions, depression, bipolar disorder, and schizophrenia. Vitamin D activates receptors on neurons in regions implicated in the regulation of behavior, stimulates neurotrophin release, and protects the brain by buffering antioxidant and anti-inflammatory defenses against vascular injury and improving metabolic and cardiovascular function. Although additional studies are needed to examine the impact of supplementation on cognition and mood disorders, given the known health benefits of vitamin D, we recommend greater supplementation in older adults.