Ameloblastic fibrosarcoma (AFS) is a rare malignant odontogenic tumour regarded as the malignant counterpart of ameloblastic fibroma. It is characterized by a benign epithelial component within a malignant fibrous stroma. AFS is a locally aggressive neoplasm with extremely low potential for metastasis. We report an extremely rare, rapidly progressive, and fatal case originating in the posterior mandible of a 20-year old female patient. Initially histopathologically diagnosed as a benign lesion, it rapidly recurred with apparent transformation into a high-grade sarcoma over a period of 6 months. Subsequent intracranial and pulmonary metastases were noted, and the patient died within 15 months of initial consultation. This case emphasizes the need for a high element of suspicion about clinically ambiguous lesions. We recommend more extensive or radical, primary excisions in lesions that have a known potential for recurrence or malignancy.