The potential for mouthguards to change the risk of concussion was studied in football helmet impacts. The Hybrid III head was modified with an articulating mandible, dentition, and compliant temporomandibular joints (TMJ). It was instrumented for triaxial head acceleration and triaxial force at the TMJs and upper dentition. Mandible force and displacement were validated against cadaver impacts to the chin. In phase 1, one of five mouthguards significantly lowered HIC in 6.7 m/s impacts (p = 0.025) from the no mouthguard condition but not in 9.5 m/s tests. In phase 2, eight mouthguards increased HIC from +1 to +17% in facemask impacts that loaded the chinstraps and mandible; one was statistically higher (p = 0.018). Peak head acceleration was +1 to +15% higher with six mouthguards and 2-3% lower with two others. The differences were not statistically significant. Five of eight mouthguards significantly reduced forces on the upper dentition by 40.8-63.9%. Mouthguards tested in this study with the Hybrid III articulating mandible lowered forces on the dentition and TMJ, but generally did not influence HIC or concussion risks.