Purpose Herpes zoster (HZ) is an acute viral eruption caused by the reactivation of varicella zoster virus (VZV), a herpes virus causing chicken pox in children. We aimed to report a 3-month neglected case of acute herpes zoster-induced third nerve palsy and optic neuritis, followed by a late-onset keratouveitis in an immunocompetent young adult. Observations A 36-year old immunocompetent Egyptian male patient presented with 3-month complaints of blurred vision and drooping of his left upper eyelid that appeared 4 days after a herpetic rash. He had been diagnosed with herpes zoster ophthalmicus (HZO) of the left eye. However, he had not received any systemic antiviral treatment. The patient had an abnormal head posture with post-eruptive scars on the left forehead and the nose tip. Examination revealed weakness of elevation and adduction, partial ptosis, and mid-dilated non-reactive pupil in the left eye. A relative afferent pupillary defect (RAPD) was present in the affected eye. His blood sugar and blood pressure were within normal limits. Contrast magnetic resonance imaging (MRI) showed no space-occupying lesion. However, there were enhancement and enlargement of the left optic nerve on T1-weighted images, denoting optic neuritis. A diagnosis of acute left third nerve palsy with pupil involvement and optic neuritis secondary to HZO was made. Despite late treatment with oral acyclovir and prednisolone, the patient recovered. One and a half months later, he developed a late-onset keratouveitis about 8 months after the rash onset. After the resolution of the episode, oral acyclovir was continued at a prophylactic dose (400 mg BID). Conclusions and importance HZ is a rare cause of third nerve palsy with pupil involvement and optic neuritis. Oral acyclovir and steroids were effective in the delayed treatment in this case. Abnormal optic nerve enhancement on MRI 3 months after the appearance of vesicular rash may suggest chronic HZ activity. Concurrent optic neuritis and third cranial nerve palsy in the absence of other signs of orbital apex syndrome can be seen in cases of HZO. Regular follow-up of patients with HZ is important for detecting recurrence and initiating prompt treatment.