Pleomorphic xanthoastrocytoma (PXA) is a rare tumour comprising <1% of all primary central nervous system tumours and the majority (~98%) occur supratentorially. We report on a 40-year-old female with a past medical history of a rare posterior fossa/cerebellar PXA who presented with a right-sided neck mass, decreased shoulder power and longstanding right tongue deviation with right-sided hemi-atrophy. The patient had prior tumour debulking. Recent MRI demonstrated an enhancing posterior fossa mass extending to the skull base at the jugular foramen and another mass in the upper neck along the jugular bulb with displacement and encasement of the right common carotid artery down to C5. Resection of the neck mass reveals an anaplastic PXA. The tumour has close approximation with adjacent peripheral nerves and is positive in 2 lymph nodes. Comparison with the original tumour molecular and immunohistochemical profiles reveals a conserved BRAF V600E mutation but the transformed malignant glioma now expresses dot-like EMA positivity and ATRX is completely lost (mutated). Transformation of a PXA (WHO Grade II) into an anaplastic PXA (WHO Grade III) has been well documented, but extracranial extension is extraordinarily rare. We report herein the first documented case of a posterior fossa PXA that underwent malignant transformation and extracranial invasion to the parapharyngeal space.