In this article I examine the advancement of therapeutic penality in the UK, a penal philosophy that reimagines prison policy, practices and environments utilising psychological knowledge. Adopting a historical approach, I show how modern therapeutic penality is linked to the emergence of personality science in the nineteenth century and the development of the democratic therapeutic community (DTC) model in the twentieth century. I outline how at the turn of the twenty‐first century a catalytic event generated a moral panic that led the British government to mobilise psychological knowledge and technologies in an attempt to manage dangerous people with severe personality disorder. Tracing subsequent developments, I argue psychological ways of talking, thinking and acting have obtained unparalleled salience in domains of penality and, in turn, radically transformed the conditions of imprisonment.