Background Although it is well established that prisoners commonly have histories of childhood trauma, little is known about mediators between exposure to trauma and criminal behaviour. Hypotheses We hypothesised that the experience of trauma in adulthood, post‐traumatic stress disorder (PTSD) and emotional dysregulation would mediate the relationship between childhood traumatic events and later criminal behaviour. Methods Eighty‐nine female prisoners were interviewed using standardised scales, in a cross‐sectional study design. History of traumatic events, DSM‐5 PTSD and emotional regulation were assessed, along with offending and demographic information. A series of regression and mediation analyses were undertaken on the data. Results Almost all (91%) of the 89 women reported both childhood and adulthood trauma. Over half (58%) met the criteria for DSM‐5 PTSD. Multiple traumas were significantly associated with seriousness of offence, as indicated by sentence length. Adult experience of trauma was the only significant mediator between childhood trauma and subsequent offending. Conclusions/implications for practice Women who have experienced multiple traumatic events may be more likely to commit serious offences, so it is very important to assess and meet their trauma‐related needs. While prisons should never be used as substitutes for healthcare facilities, when women or girls are sent to prison, the opportunity for constructive interventions must be seized. Copyright © 2017 John Wiley & Sons, Ltd.