Giant-cell tumors of bone (GCTBs) are benign, expansile lesions usually found in the long bones of young adults. The authors report a pediatric skull base GCTB causing cranial nerve deficits and vision loss. The patient presented with blurry vision, left eye misalignment, headaches, nausea, and vomiting. Imaging revealed a large heterogenous mass invading the anterior cranial fossa floor. Initial histopathology suggested an aneurysmal bone cyst (ABC), and the mass was surgically resected. Four months later, recurrence led to genetic testing, which identified a p.G35W H3F3A mutation, confirming GCTB with secondary ABC of the pituitary fossa causing right compressive optic neuropathy and left orbital apex syndrome. Management included repeat resection and denosumab therapy, achieving radiographic stability with preservation of vision in the right eye, although the optic nerve of the left eye remained atrophic. Given the diagnostic overlap between GCTB and ABC, the authors emphasize the importance of genetic testing for accurate and early diagnosis to enable timely treatment and reduce the risk of recurrence.