One of the three pillars of the Criminal Justice System in Nigeria is the Nigerian Correctional Service (NCS). Incarceration of offenders is one of the ways of managing crimes in society. In the process of incarceration, the demands of adjusting to the prison system affect the mental well-being of inmates. The study explored the mental health encounter of inmates serving at the Federal Prison-Oko, Bening City, Nigeria. An Interpretative phenomenological approach (IPA) led the way in the analysis of the qualitative data collected from 10 inmates. The researchers engaged the use of Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) and the International Classification of Diseases 11th Revision (ICD-11) mental disorder symptomologies to establish the participants ‘narration of their experiences. A feeling of hopelessness and sadness that has affected their mental state of mind were reported by participants who called for the establishment of mental health treatment centers across the country. Key Words: Federal Prison Oko-Benin City, DSM-5, ICD-11, Mental health, Criminal Justice System.