A nationwide cohort study has demonstrated a high mortality rate amongst people transferred from prison to psychiatric hospital care in New Zealand between 2009 and 2022. Of the cohort, 85% were male, 55% were Māori and median age was 31.2 years. A psychotic disorder was present in 74% and bipolar disorder in 11%, and coexisting substance use disorder was common. Median follow-up duration was 7.5 years after the first transfer; 17% of this time was spent in a psychiatric hospital. The mortality rate was 4.7-fold higher in this cohort compared with the New Zealand population, after adjustment for age and sex. Where cause of death was known, 60% were from natural causes and 40% were from injuries including suicide.