Traumatic brain injury (TBI) is a common cause of pervasive and long lasting disability and injury. The outcomes of TBI in young children can be particularly detrimental, due to the impact on both current functioning and the ability to acquire new skills. Preschool children are at heightened risk of sustaining TBI. The impact of TBI can cause difficulties across a many domains including: cognitive and intellectual ability; academic achievement; executive function (EF); adaptive and behavioural functioning; and social competence. Deficits in any of these areas can place a child at increased risk of difficulties across their transition to formal schooling, which can subsequently lead to problems in later school performance. The first aim of this study was to examine the impact of TBI on cognitive and behavioural functioning in preschool children, at 12 and 24 months post-TBI. A second aim was to compare functional outcomes for children 24 months post-TBI, compared to a control group. A population-based sample of 15 children, who sustained TBI at the age of 4 years old, were followed up over 24 months post-injury. The vast proportion of these children had sustained mild TBI. Parent ratings of behaviour, along with child performance measures of cognitive functioning were explored at 12 and 24 months post-TBI. The children with TBI were then compared to a community recruited, age-matched group of children (n = 15) at 24 months post-TBI. Parent and teacher reports of behavioural and adaptive functioning, EF, and social competence were examined, along with the children’s self-report of behavioural functioning. Child performance measures of cognitive, intellectual and academic performance were also compared across the two groups. The results showed that both behavioural and adaptive difficulties decreased from 12 to 24 months after TBI, with internalising problems showing the greatest decline. Cognitive functioning remained stable and within the average range over this time. Comparisons between the TBI and Control group at 24 months post-TBI found comparable mean scores across nearly all measures and domains. The TBI group obtained lower scores (marginally significant) on measures of estimated IQ compared to the control group. While not statistically significant, a high proportion of the TBI group had elevated scores for externalising behaviours, peer problems and overall social difficulties. This highlights the need to screen for behavioural and social difficulties, with early intervention where necessary, to reduce the risk of difficulties during the school transition. Further longitudinal research on early TBI is recommended to explore these areas further.