Severe traumatic brain injury (TBI) in older age is associated with high rates of mortality. However, little is known about outcome following mild TBI (mTBI) in older age. We report on a prospective cohort study investigating 3 month outcome in older age patients admitted to hospital-based trauma services. First, 50 mTBI older age patients and 58 orthopedic controls were compared to 123 community control participants to evaluate predisposition and general trauma effects on cognition. Specific brain injury effects were subsequently evaluated by comparing the orthopedic control and mTBI groups. Both trauma groups had significantly lower performances than the community group on prospective memory (d=0.82 to 1.18), attention set-shifting (d=-0.61 to -0.69), and physical quality of life measures (d=0.67 to 0.84). However, there was only a small to moderate but non-significant difference in the orthopedic control and mTBI group performances on the most demanding task of prospective memory (d=0.37). These findings indicate that, at 3 months following mTBI, older adults are at risk of poor cognitive performance but this is substantially accounted for by predisposition to injury or general multi-system trauma.