Abstract Persons with intellectual disabilities (ID) face pronounced health disparities. The aim of this study was to describe premature mortality by causes of death and avoidable mortality among persons with ID compared to the general Danish population. This study is based on a Danish nationwide cohort of adults (aged 18–74 years) with ID (n = 57 663) and an age- and sex-matched reference cohort (n = 607 097) which was established by linkage between several registers. The cohorts were followed in the Register of Causes of Death between 2000 and 2020. Causes of death were categorized into preventable, treatable, or unavoidable deaths using the OECD/Eurostat classification and furthermore categorized into specific interventions. We compared the observed and expected number of deaths by calculating standardized mortality ratio (SMR). Among persons with ID the number of deaths was 9400 whereof 5437 (58%) were avoidable. SMR for preventable deaths, e.g. by reducing smoking and alcohol intake or by vaccination, was 2.62 (95% CI, 2.51–2.73), and SMR for treatable deaths, e.g. by earlier diagnosis and treatment, was 6.00 (5.72–6.29). Unavoidable mortality was also six-fold increased (SMR = 6.03; 5.84–6.22). Preventable deaths were higher for persons with mild ID compared to severe ID, while treatable and unavoidable mortality were highest for persons with severe ID. The study confirmed that persons with ID have an amplified risk of mortality across all categories. There is a need for competence development of social care and healthcare personnel and reasonable adjustment of health promotion programs and healthcare services for people with ID.