Mucoceles of the paranasal sinus commonly involve the frontal sinuses, the ethmoid sinuses, and rarely the maxillary or sphenoid sinuses. They often present with sinus pain or pressure, but rarely can present with more severe symptoms such as changes in mental status or vision due to expansion and invasion through the skull base or orbit. A 62-year-old male presented with optic neuropathy, a relative afferent pupillary defect with proptosis and lateral gaze palsy of the left eye. The patient was found to have a large mucocele extending from the left posterior ethmoid sinus into the left orbital apex. Urgent endoscopic sinus surgery was performed jointly between Oculoplastics and Otolaryngology. Post-operatively, the patient had improvement in diplopia, extraocular motion, and proptosis with stable vision. This case demonstrates the importance of early identification and intervention in a rare presentation of a sinus mucocele to prevent serious complications such as vision loss.