Mucinous carcinoma of the breast, also known as colloid carcinoma, is a less common variant of breast cancer constituting less than five per cent of breast cancers. We report the case of a 42-year-old premenopausal female who presented with a palpable chest wall recurrence 4 years after simple mastectomy, axillary node dissection, and TRAM flap reconstruction for pure mucinous carcinoma. The recurrent neoplasm was a pure mucinous carcinoma and was found to be invading the mediastinum into the great vessels. The tumor was estrogen receptor positive, progesterone receptor negative, and HER-2/neu positive, which is an unusual finding for mucinous carcinoma. The fact that this tumor demonstrated HER-2/neu positivity may explain the uncharacteristic aggressive nature of this normally indolent type of breast tumor. To our knowledge, this is the first reported case of any mucinous breast cancer invading the mediastinal great vessels and its subsequent en-bloc resection.