Malignant peripheral sheath tumor (malignant PNS tumor) is a rare, aggressive, and lethal orbital neoplasm that usually arises from sensory branches of the trigeminal nerve in adults. Malignant PNS tumor spreads along nerves to involve the brain, and most patients die within 5 years of clinical diagnosis. The clinical history and examination, imaging studies, surgical procedures, and pathologic specimens of a 23-year-old man with rapid, posttraumatic onset of left orbital proptosis and pain are reported. Biopsy specimens of the tumor were compared with specimens from the same orbit obtained 18 years previously. Histopathology of the left orbital tumor compared with that of an incomplete excision of a tumor 18 years previously in the same orbit revealed both to be malignant PNS tumor. The patient is alive without recurrence 8 months after orbital exenteration and 18 years after initial tumor presentation. This is the youngest age of occurrence and the longest documented survival of a patient with orbital malignant PNS tumor.