Isolated left common carotid artery (ILCCA) is a rare cardiovascular malformation. Its core imaging feature is the anomalous connection of the left common carotid artery (LCCA) to the pulmonary artery, rendering it the rarest subtype of right aortic arch malformation. Reversed blood flow in the LCCA has been identified in the vast majority of reported cases. This malformation induces a left-to-right shunt via the common carotid artery and the circle of Willis, which subsequently leads to LCCA-steal syndrome. To date, no studies have reported the composition of this left-to-right shunt nor the neurological complications secondary to LCCA-steal syndrome. This case report elucidates the composition of the left-to-right shunt and the underlying mechanism by which the malformation elicits the steal syndrome.