Background: To assess, among people with severe mental illnesses (SMI), whether mortality risk in ethnic minority groups, relative to White British people, is modified by ethnic density. In addition, to investigate whether neighbourhood deprivation, urbanicity and social fragmentation are associated with elevated mortality in SMI.Methods: Cohort study with linked data on deaths and areas of residence, comprising 18201 individuals with an SMI diagnosis from January 1st 2007 to December 31st 2014, identified from the case-registry of a large secondary mental healthcare Trust covering an urbanised, ethnically diverse location in London, UK. Outcomes: There were 1767 deaths from all causes, 1417 from natural causes and 192 from unnatural causes. In the least ethnically dense areas, the adjusted Rate Ratio (aRR) for all-cause mortality in ethnic minority groups with SMI compared with White British people with SMI were similar, however in the highest ethnic density areas ethnic minority groups with SMI had a lower risk of death (aRR: 0.52 (95% CI: 0.38,0.71; p<0.0001), with similar trends for natural-cause mortality. In the SMI cohort, residency in deprived, urban and socially fragmented neighbourhoods was not associated with higher mortality rates. Compared with the general population, age- sex-standardised mortality ratios were elevated in the SMI cohort across all neighbourhood-level characteristics assessed.Interpretation: For ethnic minority groups with SMI, residency in areas of higher own-group ethnic density is associated with lower mortality compared to White British groups with SMI.