To compare the prevalence of multimorbidity and its impact on mortality among Aboriginal and non-Aboriginal Australians who had been hospitalised in New South Wales in the previous 10 years.Cohort study analysis of linked NSW hospital (Admitted Patient Data Collection) and mortality data for 5 437 018 New South Wales residents with an admission to a NSW hospital between 1 March 2003 and 1 March 2013, and alive at 1 March 2013.Admissions for 30 morbidities during the 10-year study period were identified. The primary outcome was the presence or absence of multimorbidity during the 10-year lookback period; the secondary outcome was mortality in the 12 months from 1 March 2013 to 1 March 2014.31.5% of Aboriginal patients had at least one morbidity and 16.1% had two or more, compared with 25.0% and 12.1% of non-Aboriginal patients. After adjusting for age, sex, and socio-economic status, the prevalence of multimorbidity among Aboriginal people was 2.59 times that for non-Aboriginal people (95% CI, 2.55-2.62). The prevalence of multimorbidity was higher among Aboriginal people in all age groups, in younger age groups because of the higher prevalence of mental morbidities, and from age 60 because of physical morbidities. The age-, sex- and socio-economic status-adjusted hazard of one-year mortality (Aboriginal v non-Aboriginal Australians) was 2.43 (95% CI, 2.24-2.62), and 1.51 (95% CI, 1.39-1.63) after also adjusting for morbidity count.The prevalence of multimorbidity was higher among Aboriginal than non-Aboriginal patients, and this difference accounted for much of the difference in mortality between the two groups. Evidence-based interventions for reducing multimorbidity among Aboriginal and Torres Strait Islander Australians must be a priority.