Background: Clinicians typically rely on neuropsychological and balance tests to track concussion recovery. The aforementioned tests imply impairments that are based on performance, but these tests do not directly measure brain physiology throughout concussion recovery. Because of these issues, an objective biomarker that can index severity and the recovery timeline is needed. Moreover, the number of concussions occurring at a recreational level requires the biomarker to be easily administered in a cost effective manner, and the results easily interpreted. Methods: To address these issues, near-infrared spectroscopy was used to assess the relative changes in oxy (HbO2)- and deoxyhaemoglobin and the associated standard deviations (SD) in the prefrontal cortex. Resting haemoglobin, and haemoglobin changes in response to hypercapnia (five repeated 20s breath holds), was measured in all participants. Data were aggregated into healthy baselines (n = 115), and concussed participants on days 1–3 (n = 14), 4–6 (n = 8), and 7–14 (n = 11). The data were statistically compared using a 1 × 4 ANOVA. Results: Resting HbO2 values progressively lowered from days 1–3 to 7–14 (with no differences compared to controls). The second major finding showed that hypercapnic HbO2 SD was significantly lower than resting values on days 1–3 and 4–6, but reversed back towards the healthy control group on day 7–14. Conclusion: Monitoring cerebral oxygenation changes is a viable biomarker to assess the physiological state of the brain following concussion.