This study aimed to determine the prevalence and predictors of poor 3 and 12 month quality of life outcomes in a cohort of pediatric patients with isolated mild TBI. We conducted a prospective cohort study of children and adolescents <18 years of age treated for an isolated mild TBI, defined as "no radiographically apparent intracranial injury" or "an isolated skull fracture, and no other clinically significant non-brain injuries." The main outcome measure was the change in quality of life from baseline at 3 and 12 months following injury, as measured by the Pediatric Quality of Life index (PedsQL). Poor functioning was defined as a decrease in total PedsQL score of >15 points between baseline and follow-up scores (at 3 and 12 months). Of the 329 patients who met inclusion criteria, 11.3% (95% CI 8.3-15.3%) at 3 months and 12.9% (95% CI 9.6-17.2%) at 12 months following injury had relatively poor functioning. Significant predictors of poor functioning included less parental education, Hispanic ethnicity (at 3 months following injury, but not at 12 months); low household income (at 3 and 12 months), and Medicaid insurance (at 12 months only). Children and adolescents sustaining a mild TBI who are socioeconomically disadvantaged may require additional intervention to mitigate the effects of mild TBI on their functioning.