A recent meta-analysis documented a significant statistical association between mild traumatic brain injury (mTBI) and attention deficit hyperactivity disorder (ADHD) (Adeyemo et al., 2014), but the direction of this effect was unclear. In this study, we hypothesized that ADHD would be an antecedent risk factor for mTBI. Participants were student athletes ages 12 to 25 who had sustained a mTBI and Controls of similar age and sex selected from studies of youth with and without ADHD. Subjects were assessed for symptoms of ADHD, concussion severity, and cognitive function. mTBI subjects had a significantly higher rate of ADHD than Controls, and in all cases the age of onset of ADHD was before mTBI onset. mTBI+ADHD subjects also had more severe concussion symptoms (fatigue and poor concentration) than mTBI-ADHD subjects. These results support ADHD as an antecedent risk factor for mTBI in student athletes and that its presence complicates the course of mTBI.