Abstract: In 2022, the U.K. Ministry of Justice started fast-tracking veterans into the prison service to reduce the violence in British prisons. These ambitions rest on a fragile evidence base. Not much is known about ex-military prison personnel. Research on the psychological effects of previous military service remains entirely absent. Drawing on 38 semistructured interviews and observations in five English prisons and using Braun and Clarke's reflexive thematic analysis, this study explores how deployment experiences influence ex-military prison personnel in their behavior and attitudes at work. In doing so, it expands our understanding of how external factors such as military service impact prison staff's approach to their work and relationship to individuals in prison. Almost all participants reported increased levels of resilience which helped them stay calm during difficult incidents at work. However, my findings show that this resilience was not always authentic nor generalizable to all contexts of their lives. Over 30% of participants suffered from posttraumatic stress symptoms, especially in periods of rest. Additionally, repeated exposure to death and injuries during military deployments desensitized them to violence, leading to emotional disengagement from individuals in prison and a strict focus on discipline. Lastly, my findings show that deployment experiences can lead to a lack of empathy, especially regarding people in prison who self-harm. I discuss policy implications around mental health support for ex-military prison staff in the prison service and adjustments to prison officer training.