Background: Prisoners are at increased risk of poor mental health and self-harming behaviours, with suicide being the leading cause of death in custody. Adverse childhood experiences (ACEs) such as child maltreatment are strong predictors of poor mental health and wellbeing yet despite high levels of ACEs in offender populations, relatively few studies have explored the relationships between ACEs and prisoners’ mental health and wellbeing. We conducted an ACE survey with 468 male adult prisoners in a Welsh prison who were not currently considered to be at risk of self-harm and suicide and explored relationships between ACEs, lifetime mental illness diagnosis, self-harm (lifetime and lifetime in prison) or suicide attempt (lifetime and lifetime in prison), and current low mental wellbeing. Results: Most participants (84.2%) had suffered at least one ACE and 45.5% had suffered ≥4 ACEs. Prevalence of lifetime mental illness diagnosis, self-harm (lifetime and lifetime in prison) or suicide attempt (lifetime and lifetime in prison), and current low mental wellbeing increased with exposure to ACEs. For example, 2.7% of those with no ACEs reported lifetime self-harm or suicide attempt in prison compared with 31.0% (self-harm in prison) and 18.3% (suicide attempt in prison) of those with ≥4 ACEs. Compared with participants with no ACEs, those with ≥4 ACEs were four times more likely to report lifetime mental illness diagnosis and suicide attempt, and over 10 times more likely to report lifetime self-harm than those with no ACEs. Independent of lifetime mental illness diagnosis, selfharm or suicide attempt, participants with ≥4 ACEs were almost three times more likely to have current low mental wellbeing than those with no ACEs.