Ethmoidal sinus mucoceles are benign expansile lesions that may progressively invade the orbit causing optic nerve compression and its nearby structures. We report a rare case of primary ethmoidal sinus mucocele instigating orbital apex syndrome. A 40-year-old man presented with right eye (RE) progressive blurring of vision with diplopia for 2 weeks. It was preceded by right-sided facial pain for 3 months. Clinical examination revealed RE proptosis with multiple cranial nerves palsy involving right cranial nerves II, III, IV, V, and VI, suggestive of right orbital apex syndrome. Magnetic resonance imaging (MRI) demonstrated right eye proptosis and right ethmoidal mucocele with intracranial and right intraorbital extension compressing the right medial rectus and optic nerve. The patient underwent an uncomplicated endoscopic sinus surgery resulting in a return to normal appearance and function post-operation. Thus, ethmoidal mucoceles are benign and curable with early recognition and intervention.