Adolescence is a risk period for offending and for traumatic brain injury (TBI) and TBI is a risk factor for poor mental health and for offending. TBI has been largely neglected from guidance on managing the mental health needs of young offenders. We sought to determine the rate of self-reported TBI, of various severities, in a male, adolescent youth offending population. We also aimed to explore whether TBI was associated with number of convictions, violent offending, mental health problems and drug misuse. Young male offenders aged 11 to 19 years were recruited from a Young Offender Institute, a Youth Offending Team and a special needs school. A total of 197 participants were approached and 186 (94.4%) completed the study. They completed self-reports on TBI, crime history, mental health and drug use. TBI with loss of consciousness (LOC) was reported by 46% of the sample. LOC consistent with mild TBI was reported by 29.6%, and 16.6% reported LOC consistent with moderate to severe TBI. Possible TBI was reported by a further 19.1%. Repeat injury was common - with 32% reporting more than one LOC. Frequency of self-reported TBI was associated with more convictions. Three or more self-reported TBIs were associated with greater violence in offences. Those with self-reported TBI were also at risk of greater mental health problems and of misuse of cannabis. TBI may be associated with offending behaviour and worse mental health outcomes. Addressing TBI within adolescent offenders with neurorehabilitative input may be important for improving well-being and reducing re-offending.