Post-traumatic hydrocephalus (PTH) is a commonly encountered complication following decompressive craniectomy, and is usually characterized by symptoms including headache, nausea, vomiting, and papilledema. Extracranial herniation accompanied by hemiplegia is a rare complication in patients with PTH who underwent craniectomy after subdural hematoma removal. We report a case of PTH that presented with extracranial herniation within one month of decompressive craniectomy. Following ventriculoperitoneal shunt implantation, left hemiplegia improved dramatically with restoration of the left middle cerebral artery blood flow, which was evident on serial imaging. Vascular compromise is often overshadowed by increased intracranial pressure when clinicians are dealing with traumatic brain injury patients. Delicate neurological and radiological examinations and prompt early interventions could lead to optimal outcomes in patients receiving decompressive craniectomy.