The SEP-MD study is a linked cohort created to explore the relationships between neighbourhood and individual factors and mortality, inpatient admissions and long-term unemployment in people with severe mental health conditions. Using individual data from the 2011 United Kingdom Census, combined with clinical records from the South London & Maudsley Trust, and death registrations, the study created a linked cohort which includes almost 20,000 individuals with a diagnosis of severe mental illness. Participants are followed up through routinely collected clinical, census and mortality data, which currently spans from 2007 to 2019.