Mortality disparities among persons with intellectual disability are important to guide health-care practices. The objective was to evaluate mortality patterns of persons with intellectual disability in a nationwide study from 1976 to 2020. This study establishes a Danish nationwide cohort of persons with intellectual disability and age- and sex-matched reference cohort through linkage between several registers. We established a cohort of 79,114 persons with intellectual disability. Standardized mortality ratios were increased for persons with intellectual disability, most pronounced among younger persons and among females. Life expectancies were markedly lower; among persons with intellectual disability 63.6 years among females and 59.8 years among males in 2016-2020 compared to 82.4 and 78.7 years among females and males in the reference cohort. Life expectancies decreased with severity of intellectual disability. This study reports the establishment of a nationwide Danish cohort of persons with intellectual disability.