Lay summary – Traumatic brain injury may increase the risk of brain diseases later in life, but it is not yet possible to tell who is most at risk. This study used UK Biobank data to compare people with and without a history of TBI, looking at thinking skills and brain scan measures of white matter, which helps different parts of the brain communicate. People with TBI showed poorer thinking performance and signs of white matter changes. These measures may help identify people at higher risk of future neurodegenerative disease, so prevention or support could be targeted earlier.