The occurrence of postconcussive symptoms (PCS) following mild traumatic brain injury (TBI) in children may depend on cognitive reserve capacity. This prospective, longitudinal study examined whether the relationship between mild TBI and PCS is moderated by cognitive ability, which served as a proxy for cognitive reserve. Participants included 182 children with mild TBI and 99 children with orthopedic injuries (OI), ranging from 8 to 15 years of age when injured. Mild TBI were classified as complicated (n = 32) or uncomplicated (n = 150) depending on whether they were associated with trauma-related intracranial abnormalities on magnetic resonance imaging. PCS were assessed initially within 3 weeks of injury, and again at 1, 3, and 12 months post injury. The initial assessment also included standardized tests of children's cognitive skills and retrospective parent ratings of pre-injury symptoms. Hierarchical linear modeling indicated that ratings of PCS were moderated jointly by cognitive ability and injury severity. Children of lower cognitive ability with a complicated mild TBI were especially prone to cognitive symptoms across time according to parents and to high acute levels of PCS according to children's self-ratings. Cognitive reserve is an important moderator of the outcomes of mild TBI in children and adolescents.