Background: Carotid atherosclerosis is a significant risk factor for stroke and has been associated with cognitive decline and dementia. Methods: We assessed 554 community-dwelling subjects from the Lothian Birth Cohort of 1936 (LBC1936) who underwent brain MRI and carotid Doppler ultrasound studies at age 73 years. The relationship between carotid stenosis and cerebral cortical thickness was examined cross-sectionally, controlling for gender, extensive vascular risk factors (VRFs), and IQ at age 11 (IQ-11). The association between carotid stenosis and a composite measure of fluid intelligence was also investigated. Results: A widespread negative association was identified between carotid stenosis and cerebral cortical thickness at age 73 years, independent of the side of carotid stenosis, other carotid measures, VRFs, or IQ-11. This association increased in an almost dose-response relationship from mild to severe degrees of carotid stenosis. A negative association was also noted between carotid stenosis and fluid intelligence, which appeared partly mediated by carotid stenosis-related thinning of the cerebral cortex. Conclusions: Carotid stenosis is associated with thinner cerebral cortex and lower fluid cognitive abilities at age 73. The findings suggest that carotid stenosis represents a marker of vascular processes that accelerate cortical aging with a negative impact on cognition, independent of measurable VRFs.