Trismus is a well-known complication of head and neck cancer treatment. It is defined as a progressive tonic contraction of the muscles of mastication that results in decreased mouth opening. This condition can lead to impairment of speech and eating, malnutrition, poor oral hygiene, and difficulty with dental treatment. Its prevalence in patients with head and neck cancer ranges from 5% to 38%. Different treatments are available to improve muscle length and function. Mouth opening devices along with exercising of the mandible immediately after surgery and/or radiation therapy have been found to be effective in reducing the trismus induced by cancer therapy. Presently, only limited defined guidelines are available for initiating or monitoring trismus therapy in this patient population. This clinical report presents a patient with head and neck cancer and a history of progressive recurrent trismus as a sequela of extensive surgery and chemoradiation, who experienced a pathological fracture of the mandible during treatment with a mouth opening device.