Current evidence suggests the possibility that autistic people may be at more risk of COVID-19 infection, hospitalisation, and mortality than the general population. Previous studies, however, are either limited in scale or do not investigate potential risk factors. Research into risk factors focused on general population samples. The current study aims to investigate these risk factors in the autistic population. Using data-linkage and a whole-country population, this study modelled associations between autism and COVID-19 hospitalisation and mortality risk in adults, investigating a multitude of clinical and demographic risk factors. Autistic adults had higher rates of hospitalisation, Standardised Incident Ratio 1.6 in 2020 and 1.3 in 2021, and mortality, Standardised Mortality Ratio 1.52 in 2020 and 1.34 in 2021, due to COVID-19 than the general population. In both populations, age, complex multimorbidity and vaccination status were the most significant predictors of COVID-19 hospitalisation and mortality. Effects of psychotropic medication varied by class. Although similar factors exhibited a positive association with heightened risk of severe COVID-19 in both the autistic and general populations, with comparable effect sizes, mortality rates were elevated among the autistic population compared to the general population. Specifically, complex multimorbidity and classification of prescribed medications may emerge as particularly significant predictors of severe COVID-19 among individuals within the autistic population due to higher prevalence of complex multimorbidity in the autistic population and variability in the association between medication classes and severe COVID-19 between both populations, though further research is needed.