The aim of this study was to determine the prevalence of mental disorders among offenders admitted at the health establishments in Bizzah Makhathe Correctional Centre, South Africa. Structured psychiatric interviews were conducted to elicit the information. The majority of the participants were young male Africans with low educational levels and poor socio-economic status. Crimes against human beings were the most common ones committed by the offenders. The lifetime prevalence of mental disorders was 54.7%. Personality disorders, followed by substance and addictive disorders, were the most prominent disorders among the study sample. Other psychiatric disorders noted were depressive disorders, schizophrenia, intellectual disabilities and neurocognitive disorders. The results show that mental disorders in prisons go undetected and untreated, meaning that competency to stand trial and criminal responsibility are not always observed. There is a need to conduct more empirical studies on the prevalence and incidences of mental disorders in South African prisons.