Horner's syndrome is characterized by a classic triad of ipsilateral pupillary miosis, partial eyelid ptosis, and facial anhydrosis. This case study reports a 7-year-old boy with right miosis, mild blepharoptosis, and iris hypopigmentation detected in a routine pediatric follow-up without ipsilateral facial anhydrosis, flushing, or pain. There was no history of birth trauma and test with cocaine provoked no response of the right pupil, suggesting right Horner's syndrome. Mediastinal tumor was ruled out and brain magnetic resonance imaging incidentally showed absence of flow in the right internal carotid artery. Subsequent magnetic resonance angiography demonstrated agenesis of the right internal carotid artery without other vascular-associated malformations. The final diagnosis was right, congenital Horner's syndrome due to ipsilateral internal carotid agenesis. We describe in detail the radiological findings and pathophysiological mechanisms of this unusual association.