Abstract Background Human rights watchdogs have described conditions in Nigerian correctional facilities and detention centers as damaging to the physical and mental health of inmates. While the prevalence of mental disorders is high, access to appropriate healthcare is grossly inadequate. Understanding the current state of prison inmates’ mental health and well-being is an essential first step to addressing this important issue. This study aims to document the mental health and experiences of incarceration of inmates of the largest medium security prison in Nigeria’s Benue State. Methods A cross-sectional survey and descriptive analysis was carried out with a random sample of 381 prison inmates of Benue State Makurdi Medium Security Prison. Survey tools included: (1) a structured questionnaire on participants’ experiences in prison, and (2) the Patient Health Questionnaire (PHQ-9), a screening tool for depression. Results Most participants were young men (95.5%, mean age 27.95) and had completed secondary school (63.5%). While prison authorities had identified only 27 participants as having a mental disorder, 144 (37.8%) screened positive for depression. Twenty six had received professional counseling while in prison. Of the six participants who were already taking a psychotropic medication at the time of imprisonment, four received medication after being imprisoned. Approximately half, (52%) of participants were dissatisfied with prison health care. Conclusions Despite the high prevalence of depression among prison inmates, few cases are detected and treated. Prison staff may not recognize depression as a mental disorder, and the mental health care available is generally poor. Inadequate mental health and social care not only affects prison inmates’ well-being, but may also impact recidivism and health outcomes upon release. Prison inmates should be screened routinely for depression and other less-commonly recognized mental health conditions, and appropriate treatment made available.