BACKGROUND: Across and within countries there is a need to understand how the COVID-19 pandemic has impacted populations of individuals with intellectual and developmental disabilities (IDD). OBJECTIVE: Rates of COVID-19 positivity for adults with IDD, including Down syndrome, relative to adults without IDD in Ontario, Canada were compared. Health profiles and case-based rates of hospitalizations, intensive care unit admissions, and mortality within 30 days of testing positively were compared for those with IDD, including Down syndrome, versus those without IDD. METHODS: This retrospective cohort study linked health administrative databases using unique encoded identifiers to describe population-level COVID-19 positivity, related hospital use and mortality from January 15, 2020 to January 10, 2021. Incidence rate ratios (RR) and 95% confidence intervals were calculated. RESULTS: Relative to adults without IDD, COVID-19 positivity rates were 1.28 times higher for adults with IDD and 1.42 times higher for adults with Down syndrome. Compared to adults without IDD, adults with IDD were more than twice as likely to be hospitalized following COVID-19 (RR:2.21 (95%CI: 1.93,2.54)) and to die (RR:2.23 (95%CI: 1.86,2.67). These RRs were greater for adults under 65. For adults with Down syndrome, mortality rates were 6.59 (95%CI: 4.51,9.62) times higher than those without IDD. DISCUSSION: In Ontario, Canada, hospitalization and mortality rates associated with COVID-19 are higher for adults with IDD than other adults. These findings should inform vaccination strategies that often prioritize older adults in the general population resulting in people with IDD, who are often in younger age groups, being overlooked.