Background: There is little research with people who experience intellectual/developmental disabilities and imprisonment. Methods: The study linked health and correctional data to examine prevalence of intellectual/developmental disabilities and health and correctional characteristics among adults experiencing their first federal incarceration between 1 January 2002 and 31 December 2011 (n = 9278) and two non-incarcerated groups (n = 10,086,802). Results: The prevalence of intellectual/developmental disabilities was 2.1% in the incarcerated group and 0.9% in the non-incarcerated group. Before incarceration, those with, versus without, intellectual/developmental disabilities were at greater risk of traumatic brain injury, mental illness, and substance use disorders. While incarcerated, those with intellectual/developmental disabilities were more likely to incur serious institutional disciplinary charges. Post-incarceration, persons with intellectual/developmental disabilities were at greater risk of emergency department visits, and psychiatric and acute hospitalizations, than the non-incarcerated groups. Conclusions: People with intellectual/developmental disabilities are overrepresented in Canadian federal correctional institutions. The authors offer strategies to support people prior to, during, and post-incarceration.