Adult-onset Alexander disease (AOAD) has been increasingly recognized since the identification of the glial fibrillary acidic protein gene mutation in 2001. We report on a 56-year-old man who was genetically confirmed as AOAD with the glial fibrillary acidic protein mutation of p.M74T. He developed spastic tetraparesis, sensory disturbances in four limbs, and mild cognitive impairment without apparent dysarthria and dysphagia. The case was characterized by severe atrophy of the medulla oblongata and upper cervical cord with intramedullary signal intensity changes on magnetic resonance imaging. While AOAD is diverse in clinical presentation, the peculiar magnetic resonance imaging findings of marked atrophy of the medulla oblongata and cervical cord are thought to be highly suggestive of the diagnosis of AOAD.