Concussion or mild traumatic brain injury (mTBI) is often accompanied by long-term behavioral and neuropsychological deficits. Emerging data suggest that these deficits can be exacerbated following repeated injuries. However, despite the overwhelming prevalence of mTBI in children due to falls and sports-related activities, the effects of mTBI on white matter (WM) structure and its development in children have not been extensively examined. Moreover, the effect of repeated mTBI (rmTBI) on developing WM has not yet been studied, despite the possibility of exacerbated outcomes with repeat injuries. To address this knowledge gap, we investigated the long-term effects of single (s)mTBI and rmTBI on the WM in the pediatric brain, focusing on the anterior commissure (AC), a WM structure distant to the injury site, using diffusion tensor imaging (DTI) and immunohistochemistry (IHC). We hypothesized that smTBI and rmTBI to the developing mouse brain would lead to abnormalities in microstructural integrity and impaired oligodendrocyte (OL) development. We used a postnatal day 14 Ascl1-CreER: ccGFP mouse closed head injury (CHI) model with a bilateral repeated injury. We demonstrate that smTBI and rmTBI differentially lead to myelin-related diffusion changes in the WM and to abnormal OL development in the AC, which are accompanied by behavioral deficits 2 months after the initial injury. Our results suggest that mTBIs elicit long-term behavioral alterations and OL-associated WM dysregulation in the developing brain. These findings warrant additional research into the development of WM and OL as key components of pediatric TBI pathology and potential therapeutic targets.