Aortic mural thrombus (AMT) in the ascending aorta is a rare source of embolism. Recently, the usefulness of contrast computed tomography (CT) has been reported, and we sought to examine the differences between cardiac CT and CT angiography (CTA). A 58-year-old patient of acute embolic infarction was treated by endovascular thrombectomy. Postoperative cardiac CT revealed the AMT as an embolic source. The lesion was not detected by the CTA performed 2 days before. This is the first case report of AMT to highlight the apparent utility of cardiac CT. Although trans-esophageal echocardiogram (TEE) is still the first choice for routine embolic exploration, cardiac CT may play a role as an alternative tool aimed to detect small size AMT.