Background Intellectually disabled (ID) patients present unique therapeutic challenges. We aimed to describe the characteristics of ID patients admitted to a general intensive care unit (ICU). Results We conducted a retrospective cohort study comparing critically ill adult ID patients to matched patients without ID (1:2 ratio) in a single ICU (2010–2020). The main outcome measure was mortality. Secondary outcomes included complications during admission and characteristics of weaning from mechanical ventilation. The study and control groups were randomly selected based on similar age and sex. ID patients nonetheless had an average APACHE score of 18.5 ± 8.7 vs. 13.4 ± 8.5 in controls (p < 0.001). ID patients had more hematological (p = 0.04), endocrinological (p < 0.001) and neurological (p = 0.004) comorbidities and used more psychiatric medication before admission. No difference was found in mortality rates. Differences were found as there were more secondary complications, such as pulmonary and sepsis (p < 0.03), frequent requirement of vasopressors (p = 0.001), significantly higher intubation rates with more weaning attempts, tracheostomies and longer ICU and hospital admissions (p < 0.019). Conclusions Critically ill adult ID may have more comorbidities and be sicker at the time of admission compared to their age- and sex-matched counterparts. They require more supportive treatment and their weaning from mechanical ventilation may be more challenging.