The degree to which postinjury posttraumatic stress disorder (PTSD) and/or depressive symptoms in adolescents are associated with cognitive and functional impairments at 12 and 24 months after traumatic brain injury (TBI) is not yet known. The current study used a prospective cohort design, with baseline assessment and 3-, 12-, and 24-month followup, and recruited a cohort of 228 adolescents ages 14–17 years who sustained either a TBI (n = 189) or an isolated arm injury (n = 39). Linear mixed-effects regression was used to assess differences in depressive and PTSD symptoms between TBI and arm-injured patients and to assess the association between 3-month PTSD and depressive symptoms and cognitive and functional outcomes. Results indicated that patients who sustained a mild TBI without intracranial hemorrhage reported significantly worse PTSD (Hedges g = 0.49, p = .01; Model R2 = .38) symptoms across time as compared to the arm injured control group. Greater levels of PTSD symptoms were associated with poorer school (η2 = .07, p = .03; Model R2 = .36) and physical (η2 = .11, p = .01; Model R2 = .23) functioning, whereas greater depressive symptoms were associated with poorer school (η2 = .06, p = .05; Model R2 = .39) functioning.