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.