Background and Objectives Previous studies estimated that modifiable risk factors explain up to 40% of the dementia cases in the United States and that this population-attributable fraction (PAF) differs by race and ethnicity—estimates of future impact based on the risk factor prevalence in contemporary surveys. The aim of this study was to determine the race-specific and ethnicity-specific PAF of late-onset Alzheimer disease and related dementias (ADRDs) based on the risk factor prevalence and associations observed on the same individuals within a prospective cohort. Methods Data were from Multiethnic Cohort Study participants (African American, Japanese American, Latino, Native Hawaiian, and White) enrolled in Medicare Fee-for-Service. We estimated the PAF based on the prevalence of risk factors at cohort baseline and their mutually adjusted association with subsequent ADRD incidence. Risk factors included low educational attainment and midlife exposures to low neighborhood socioeconomic status, unmarried status, history of hypertension, stroke, diabetes or heart disease, smoking, physical inactivity, short or long sleep duration, obesity, and low-quality diet, as well as APOE ε4 for a subset. Results Among 91,881 participants (mean age 59.3 at baseline, 55.0% female participants), 16,507 incident ADRD cases were identified from Medicare claims (1999–2016, mean follow-up 9.3 years). The PAF for nongenetic factors combined was similar in men (24.0% [95% CI 21.3–26.6]) and women (22.8% [20.3–25.2]) but varied across Japanese American (14.2% [11.1–17.2]), White (21.9% [19.0–24.7]), African American (27.8% [22.3–33.0]), Native Hawaiian (29.3% [21.0–36.7]), and Latino (33.3% [27.5–38.5]) groups. The combined PAF was attenuated when accounting for competing risk of death, in both men (10.4%) and women (13.9%) and across racial and ethnic groups (4.7%–25.5%). The combined PAF was also different by age at diagnosis and ADRD subtypes, higher for younger (65–74 years: 43.2%) than older (75–84 years: 32.4%; ≥85 years: 11.3%) diagnoses and higher for vascular or unspecified ADRD than for AD or Lewy body dementia. An additional PAF of 11.8% (9.9–13.6) was associated with APOE ε4, which together with nongenetic risk factors accounted for 30.6% (25.8–35.1) of ADRD. Discussion Known risk factors explained about a third of the ADRD cases but with unequal distributions across racial and ethnic groups.