People with severe mental illness have a reduced life expectancy of up to 15-20 years compared to the general population. There are many factors contributing to this excess mortality, with the three most common causes being cardiovascular disease, cancer and suicide. The purpose of our study was to provide an overview of the current evidence and review potential causes of this excess mortality. We conducted a non-systematic search in PubMed of studies of schizophrenia and bipolar disorder and causes of excess death based on premade inclusion criteria. This provided 92 publications, and the review confirmed an excess mortality of 20-25 years in schizophrenia and 10 years in bipolar disorder. Further, the review confirmed that side effects from antipsychotic drugs, smoking and cardiovascular disease are major causes of premature death in severe mental illness, in addition to cancer. Finally based on the results, we suggest that interventions like seeing their doctor more regularly, having a good relationship between the doctor and the patient and motivating the patient to change unhealthy lifestyle habits such as smoking and poor diet can be applied to reduce this excess mortality among these patients.