An acquired brain injury often leads to long-lasting mental fatigue, which can have a considerable effect on work and social interactions. Fortunately, the Mindfulness-Based Stress Reduction (MBSR) program has been found to alleviate mental fatigue. The purpose of this feasibility study was to evaluate the success of an interactive MBSR program delivered live online to individuals who have experienced a traumatic brain injury or stroke. We included the following three groups in our study: an Internet group, a face-to-face MBSR group, and an active control group who took weekly walks in natural environments. Thirty-four participants completed the study, and all were suffering from long-lasting mental fatigue after either a traumatic brain injury (16 participants) or a stroke (18 participants). However, seven did not accept to attend an Internet MBSR, and Internet was the only choice for others. We found that, according to the Mental Fatigue Scale (MFS), the program leads to significantly reduced mental fatigue in the Internet group compared with the face-to-face and the control group. Individuals in the MBSR groups also exhibited an improved ability to process two temporally close targets (attentional blink task), while this was not detected in the control group. In conclusion, we believe that it is possible for individuals suffering from mental fatigue after an acquired brain injury to obtain positive results through enrollment in a live, interactive, online MBSR program. This is promising, as the Internet is accessible to many individuals, irrespective of where they live. Further randomized control studies comparing are warranted.