The high prevalence of mental health problems in prisons is well established and there are ongoing challenges to mental health care and service provision. The aim of this research was to identify practice that was working well and where there was scope for improvement. Forty-two independent inspectorate reports on conditions for and treatment of people across all seven categories of prison in England and Wales were analyzed thematically. Themes of What is Working and Shortfalls identified that multidisciplinary working, good communication, balancing care with security, and positive staff-prisoner relationships supported mental health, but there were numerous shortfalls in service provision across the prison estate. Conclusions support an agenda of prison reform with mental health as a key priority.