Though clinical cardiovascular and cerebrovascular diseases are established risk factors for cognitive decline and dementia, less is known about the relations between vascular health and cognition among individuals without these diseases. Carotid intimal medial thickness (IMT), a measure of subclinical vascular disease, is associated with concurrent decrements in cognitive function, but relatively little research has examined relations between carotid IMT and prospective cognitive decline. The present investigation examined associations between carotid IMT and cognitive function, both cross-sectionally and longitudinally, among 538 participants (aged 20 to 93, 39% male, 66% white) in the Baltimore Longitudinal Study of Aging free of known cardiovascular, cerebrovascular, and neurological disease. Participants underwent initial carotid ultrasonography and repeat neuropsychological testing on up to eight occasions over up to 11 years of follow-up. Mixed-effects regression analyses were adjusted for age, gender, race, education, mean arterial pressure, body mass index, total cholesterol, smoking, depressive symptoms, and cardiovascular medication use. Significant cross-sectional associations between carotid IMT and cognitive performance were largely qualified by significant longitudinal associations. In contrast to those individuals with lesser carotid IMT values, individuals with greater carotid IMTs displayed accelerated decline in performance over time on multiple tests of verbal and nonverbal memory, as well as a test of semantic association fluency and executive function. These findings underscore the importance of early intervention to delay or reduce atherosclerosis and improve vascular health before symptom manifestation. Such intervention (e.g., pharmacological, behavioral) may delay onset of cognitive decline, slow cognitive aging, and perhaps delay or protect against clinical cognitive diagnoses.