Persons suffering from severe mental disorder have an excess mortality compared to persons with no mental disorder. However, the magnitude of the excess mortality differs from one mental disorder to another, and the impact on mortality if a first-degree family member suffers from a mental disorder has never been examined in a population-based study. Our objective was to examine and compare mortality rates after admission with schizophrenia, schizoaffective disorder, unipolar depressive disorder, or bipolar affective disorder and to examine the impact of family history of psychiatric admission on mortality. We established a register-based cohort study of 5.5 million persons born in Denmark who were alive on or born after January 1, 1973 and alive on their 15th birthday. Mortality rate ratios were estimated by survival analysis, using Poisson regression. Unipolar depressive disorder, bipolar affective disorder, and schizoaffective disorder were associated with the same pattern of excess mortality. Schizophrenia had a lower mortality from unnatural causes of death and a higher mortality from natural causes compared to the 3 other disorders. Family history of psychiatric admission was associated with excess mortality. Patients suffering from the 4 disorders all had an excess mortality, but the pattern of excess mortality was not the same. There was an excess mortality associated with mental disorder in a first-degree family member, but this only explained a small part of the general excess mortality associated with the 4 mental disorders examined.