Problem: Mild traumatic brain injury (mTBI) can result in long-term sequelae. Lack of sensitive biomarkers makes diagnosis challenging. Methods: Cross-sectional and longitudinal study of 40 mTBI patients at 2 weeks and 6 months after injury. Diffusion tensor imaging and multishell neurite orientation dispersion and density imaging (NODDI) parameters were assessed. Results: Cross-sectional analysis between patients at 2-weeks and controls revealed a decrease of fractional anisotropy and increase of mean diffusivity in the patient group together with elevated free water values. Longitudinally, after mTBI, a decline in neurite density was observed. Conclusions: NODDI measurements are sensitive imaging biomarkers for the subtle underlying white matter pathology after mTBI.