Adults on the autism spectrum and those with intellectual disability or mental health conditions may be at increased risk of contracting COVID-19 or experiencing more severe illness if infected. We identified risk factors for COVID-19 among adults enrolled in Medicaid with an autism spectrum disorder diagnosis, intellectual disability, or mental health conditions. We examined adults ages 20-64 years with 9-month continuous enrollment over 2008-2012 using Medicaid Analytic eXtract data. There were 83,150 autistic adults and 615,607 adults with intellectual disability meeting inclusion criteria; of a random sample of 1 million beneficiaries without autism spectrum disorder or intellectual disability, 35.3% had any mental health condition. Beneficiaries on the spectrum, those with intellectual disability, and those with mental health conditions all had higher odds of risk factors for becoming infected with COVID-19 (living in a residential facility, receiving services in the home from outside caregivers, having had a long hospitalization, and having had avoidable hospitalizations) and higher odds of comorbidities associated with severe illness from COVID-19. Clinicians should anticipate high prevalence of comorbidities and risk factors for severe illness from COVID-19 among these populations. Health officials and non-governmental organizations should target these groups with outreach for the COVID-19 vaccine and support continued efforts for appropriate mitigation measures.