Aim The primary aim of this study was to conduct an open pilot clinical trial of a brief mindfulness-based intervention for persistent postconcussion symptoms that occur after mild traumatic brain injury in military service members. For many service members, operational tempo and other time constraints may prevent them from completing a standard mindfulness-based stress reduction course. Thus, this study sought to examine the effectiveness of a five-session intervention called Mindfulness-Based Stress, Pain, Emotion, and Attention Regulation (MSPEAR). Methods Participants were active duty service members with a history of mild traumatic brain injury (TBI) and persisting postconcussion symptoms, all of whom were recruited from an outpatient TBI rehabilitation program at a military treatment facility. Of the 38 service members that were initially enrolled, 25 completed the 5-session MSPEAR intervention, and 20 returned for a 5-week follow-up evaluation. Questionnaires assessing perceived stress, positive affect, pain interference and catastrophizing, sleep disturbances, perceived behavioral and attention regulation, self-efficacy, and satisfaction with life were administered at pre-intervention, post-intervention, and at 5-week follow-up intervals. Neuropsychological testing at pre-intervention and 5-week follow-up included performance validity measures, attention, working memory, and executive function measures. Results Improvements in perceived stress, positive affect, behavioral regulation, metacognition, sleep disturbance, self-efficacy, and satisfaction with life were found immediately after the MSPEAR intervention and were maintained at the 5-week follow-up. Magnification and helplessness aspects of pain catastrophizing improved when comparing pre-intervention to the 5-week follow-up. Pain interference was not significantly different across study assessment times. Neuropsychological testing revealed improvements in sustained attention, working memory, cognitive flexibility, and inhibitory control when comparing pre-intervention to the 5-week follow-up assessment. Conclusions The MSPEAR intervention appears to show promise as a brief and effective therapy for specific postconcussion symptoms after mild traumatic brain injury in military service members. Each of the components of MSPEAR including stress, pain catastrophizing, emotion, and attention regulation showed improvements in this study, and bears further investigation in a larger scale, preferably randomized controlled trial in those active duty military service members who experience persisting symptoms after a mild traumatic brain injury.