Background and purpose: Mortality is known to be markedly increased in people with dementia. However, the association between multiple chronic conditions and mortality in dementia is not well clarified. The aim of this study was to investigate the impact of somatic and psychiatric diseases on mortality in dementia compared with the general elderly population. Methods: Using a cohort study design, nationwide registry data from 2006 to 2015 on dementia and psychiatric and somatic comorbidities defined by the Charlson Comorbidity Index (CCI) were linked. Impact of chronic conditions was assessed according to mortality rate ratios (MRRs) in all Danish residents aged ≥65 years with and without dementia. Results: Our population comprised 1,518,917 people, of whom 114,109 people were registered with dementia. The MRRs was 2.70 (95% confidence interval 2.68, 2.72) in people with dementia after adjusting for sex, age, calendar year, and comorbidities. MRRs increased with higher CCI score, and when comparing people with a similar comorbidity load, MRRs were significantly higher for people with dementia. Conclusions: The comorbidity load was associated with increased mortality in both people with and without dementia. Mortality in dementia remained increased, even after adjusting for psychiatric and chronic somatic comorbidities. Our findings suggest that dementia disorders alone contribute to excess mortality, which may be further increased by comorbidities.