Mycotic aneurysms most commonly occur in femoral arteries or the abdominal aorta. Mycotic aneurysm arising from the left subclavian artery is very rare. The morbidity and mortality of ruptured mycotic aneurysms, regardless of their location, remain high despite the current practice of administering an intensive antibiotic regimen. We encountered a case of mycotic aneurysm presenting as mediastinal abscess and arising from in the left subclavian artery. Therefore, we report this case with radiologic findings to remind readers of the possibility of this unusual location of mycotic aneurysm.