Mannitol is widely used to manage elevated intracranial pressure but extravasation of this hyperosmolar agent can lead to rapid soft tissue injury, compartment syndrome, and skin necrosis. Management ranges from conservative measures to urgent surgical debridement or fasciotomy, with significant associated morbidity. We report a case of extensive mannitol extravasation in a 16-year-old female undergoing urgent suboccipital craniectomy for posterior fossa haemorrhage secondary to a ruptured cerebellar arteriovenous malformation. Approximately 350 mL of mannitol extravasated via a peripheral cannula in the proximal volar forearm, resulting in rapidly progressive swelling, tense compartments, bruising, and loss of palpable distal pulses, although hand perfusion was maintained. The peripheral cannula was removed and a minimally invasive bedside approach was undertaken. Hyaluronidase (15 units diluted in saline) was injected into the affected area, followed by multiple small (5 mm) stab incisions and saline infiltration with expression to facilitate washout of the extravasated agent. The limb was dressed and elevated. The forearm improved progressively with daily review. Compartment firmness reduced, blistering resolved, and all skin remained viable. No further surgical intervention, debridement, or fasciotomy was required. This case demonstrates that early, aggressive minimally invasive management combining hyaluronidase, saline washout, and adequately sized skin incisions can successfully treat severe mannitol extravasation, preserve tissue viability, and avoid the morbidity associated with major surgical intervention. This approach represents a valuable strategy for managing hyperosmolar extravasation injuries.