Background Increased carotid intima‐media thickness, interadventitial diameter, presence of carotid plaque, and lower distensibility are predictors for cardiovascular disease. These indices likely relate to cerebrovascular disease, and thus may constitute a form of vascular contributions to dementia and Alzheimer disease–related dementia. Therefore, we assessed the relationship of carotid measurements and arterial stiffness with incident dementia in the ARIC (Atherosclerosis Risk in Communities) study. Methods and Results A total of 12 459 ARIC participants with carotid arterial ultrasounds in 1990 to 1992 were followed through 2017 for dementia. Dementia cases were identified using in‐person and phone cognitive status assessments, hospitalization discharge codes, and death certificate codes. Cox proportional hazards models were used to estimate the hazard ratios (HRs) for incident dementia. Participants were aged 57±6 at baseline, 57% were women, and 23% were Black individuals. Over a median follow‐up time of 24 years, 2224 dementia events were ascertained. After multivariable adjustments, the highest quintile of carotid intima‐media thickness and interadventitial diameter in midlife was associated with increased risk of dementia (HR [95% CIs], 1.25 [1.08–1.45]; and 1.22 [1.04–1.43], respectively) compared with its respective lowest quintile. Presence of carotid plaque did not have a significant association with dementia (HR [95% CI], 1.06 [0.97–1.15]). Higher distensibility was associated with lower risk of dementia (HR [95% CI] highest versus lowest quintile, 0.76 [0.63–0.91]). Conclusions Greater carotid intima‐media thickness, interadventitial diameter, and lower carotid distensibility are associated with an increased risk of incident dementia. These findings suggest that both atherosclerosis and carotid stiffness may be implicated in dementia risk.