Late onset seroma is a rare post-operative complication occurring after various surgeries including thymectomy. Most cases are asymptomatic; however, seromas occurring in the mediastinal cavity may cause compression symptoms including airway compression or cardiac tamponade. We present a 62-year-old male with a history of thymectomy for myasthenia gravis who presented with cardiac tamponade several years ago. Further evaluation revealed a late onset seroma anteriorly compressing the cardiac chambers resulting in tamponade physiology.