We aim to provide insight into the cause-specific mortality of older adults with intellectual disability (ID), with and without Down syndrome (DS), and compare this to the general population. Immediate and primary cause of death were collected through medical files of 1,050 older adults with ID, 5 years after the start of the Healthy Ageing and Intellectual Disabilities (HA-ID) study. During the follow-up period, 207 (19.7%) participants died, of whom 54 (26.1%) had DS. Respiratory failure was the most common immediate cause of death (43.4%), followed by dehydration/malnutrition (20.8%), and cardiovascular diseases (9.4%). In adults with DS, the most common cause was respiratory disease (73.3%), infectious and bacterial diseases (4.4%), and diseases of the digestive system (4.4%). Diseases of the respiratory system also formed the largest group of primary causes of death (32.1%; 80.4% was due to pneumonia), followed by neoplasms (17.6%), and diseases of the circulatory system (8.2%). In adults with DS, the main primary cause was also respiratory diseases (51.1%), followed by dementia (22.2%).