This study investigated the efficacy of a 10-session nature-based therapeutic intervention for people with post-concussion symptoms. The intervention involved physical and vestibular exercises, sensory training, relaxation, and psychoeducation, all of which were integrated with the natural environment in a forest therapy garden. This study was designed with a passive control period followed by the intervention (n = 30). The Mental Fatigue Scale (MFS) was the primary outcome measure. The secondary outcome measures were the Warwick–Edinburg Mental Wellbeing Scale and the short version of the Quality of Life after Brain Injury. A Likert scale was used to examine the mental strain of the sessions themselves. The MFS (primary outcome) exhibited a significant decrease with a medium-sized effect from before to after the intervention. The secondary outcomes exhibited significant increases from the beginning to the end of the intervention. All outcomes were sustained at follow-up ten weeks later. No significant difference was found from the control period. This study indicates that the described nature-based intervention is a feasible treatment for reducing prolonged post-concussion symptoms. However, it should be studied more in-depth to understand the impact of the natural environment and to validate the results on a larger representative population.