A substantial number of prisoners have intellectual disabilities. We analysed data on a sample drawn from all prisons in England and Wales. Intellectual disability was defined as Quick Test scores equivalent to an IQ of ≤65. We found a significantly higher prevalence of probable psychosis, attempted suicide and cannabis use in prisoners with intellectual disabilities. Presence of intellectual disability was twice as likely to be associated with probable psychosis but the relationship was fully mediated by self-rated health status. It is important to identify this group as early as possible in order to provide timely interventions to cope in adverse environments and manage substance misuse.