Ex-armed forces personnel constitute the largest known occupational group in prison but there is little evidence regarding their mental health, or substance misuse, needs. A total of 105 participants were interviewed and measures assessing symptoms of common mental health (CMH) problems and substance misuse were completed along with a review of their health care records. Forty (38%) participants screened for current CMH problems (CCMH) and high levels of dual symptomology and alcohol misuse were assessed. Thirty-nine (37%) had a mental health diagnosis recorded, most commonly for post-traumatic stress disorder (PTSD), depression and personality disorder. Those who screened for a CCMH problem were more likely to have pre-service vulnerability to negative health outcomes and those with dual symptomology were more likely to have experienced deployment during their service. Findings suggest the mental health needs of this group are similar to the general prison population. Potentially higher prevalences of PTSD and alcohol misuse may direct service provision.