A 67-year-old male presented with sudden headache and decreased consciousness, diagnosed as spontaneous subarachnoid hemorrhage (SAH). Imaging revealed an anterior communicating artery (ACoA) aneurysm and congenital absence of the left internal carotid artery (ICA), confirmed by the absence of the carotid canal. The aneurysm was surgically clipped. Postoperative angiography demonstrated that the left anterior and middle cerebral arteries were perfused via collateral flow from the posterior circulation. The patient made a full recovery. Congenital ICA absence is rare but significantly alters cerebral hemodynamics, especially within the circle of Willis, predisposing patients to aneurysm formation. This case underlines the importance of vascular imaging in patients with SAH and highlights the need for urgent diagnosis and intervention in the presence of congenital vascular anomalies.