Introduction: Mild traumatic brain injury (mTBI) accounts for the vast majority of all paediatric TBI cases. It is an important public health concern, yet the long-term psychiatric and behavioural outcomes remain imperfectly understood. Aim. This study aims to examine the association between paediatric mTBI and psychiatric and offending outcomes in adulthood, while considering the impact of sex, age at injury and duration since injury on outcome. Participants: Participants with mTBI (n=57) were compared to those with moderate/severe TBI (n=62) and to orthopaedic injury controls (n=42). All participants were injured at age 17 or younger and were 18 years or older at the time of assessment. Outcome measures: Based on the DSM-IV-TR criteria, structured interviews were used to assess participants’ experience of symptoms consistent with major depressive disorder, anxiety disorders (including generalised anxiety disorder, panic attacks and panic disorder, agoraphobia, social phobia, post-traumatic stress disorder, and specific phobia), and substance abuse and/or dependence. Participants’ were asked to report on their lifetime involvement with offending, arrests, and diversions and/or convictions. Results: At age 18-31, participants with a paediatric mTBI were significantly more likely than orthopaedic injury controls to endorse symptoms consistent with major depressive disorder by 3.17 times, anxiety disorders by 5.81 times, and internalising disorders in general by 5.80 times and the risk in the mTBI group was greater than that for those with moderate/severe TBI. Females with mTBI were significantly more likely than males, by five times, to endorse an internalising disorder. Paediatric mTBI was not significantly associated with externalising problems when compared with controls; however, males with mTBI were 6.57 times more likely to endorse externalising behaviours than females. Conclusions: Paediatric mTBI is a risk factor for internalising disorders in adulthood, particularly for females. Such findings have implications for assessment and treatment of problems associated with paediatric mTBI.