Sinusitis is a common condition, but only very rarely accompanied by isolated cranial nerve palsies. We describe a case of a 64-year-old male with a two-day history of left-sided ptosis associated with one week of nasal congestion and frontal sinus pain. Examination revealed ptosis with left pupil mydriasis. Uncontrasted computed tomography and angiography of the head demonstrated neither intracranial vascular abnormalities nor acute lesions; however, it did show mucosal thickening in the left frontal sinus, ethmoid air cells and left maxillary sinus, indicating potential obstruction of the left ostiomeatal complex. The sinusitis was treated with intranasal steroids, xylomethazoline and nasal douching. The patient reported resolution of all symptoms, including left ptosis, within one week of therapy. This rare case of sinusitis causing ptosis is presented due to its infrequent nature, such that awareness of the differential diagnosis of cranial nerve palsy and complications of sinusitis may be improved.