A traumatic brain injury (TBI) is one of the most common injuries sustained by adolescents and a leading cause of death and long-term disability. The vast majority of these cases are mild and yet they can cause a range of difficulties across multiple domains which can negatively impact upon a person’s social functioning. The aim of this study was to determine whether adolescents who have sustained a mild TBI would differ from their uninjured peers in regards to social competence. A secondary aim was to investigate whether there was a relationship between an adolescent’s level of social competence and their executive functioning abilities. This investigation involved a population-based sample of 30 young adolescents who had sustained a mild TBI between the age of 12 and 15 years. Parental and/or self-report ratings of behaviour and executive function, as well as a performance-based measure of cognition were explored at 12 months post-injury. These adolescents were then compared to a community recruited, age-matched sample of their TBI-free peers. A composite scale of social competence based on observable behaviours was formulated. The results showed adolescents with a mild TBI demonstrated greater difficulties with socially competent behaviours compared to their uninjured peers. These difficulties were found to centre around aspect of their behavioural functioning, particularly regarding their regulation and inhibition of behaviour. Higher levels of dysfunction in the mild TBI sample were also found for parental ratings of behaviours associated with executive functions. A relationship between the Social Competence Index and executive function was not found. However, a relationship between the Social Competence Index and composite measures of cognitive functioning was. Suggesting there are more cognitions behind socially competent behaviour than the domain of executive functions alone. These findings highlight the need for post-injury screening for behavioural and social difficulties, with interventions implemented as required, to reduce the risk of on-going social impairment.