A medullary thyroid carcinoma is a malignant tumor derived from the C-cells of the thyroid. Despite their distinct embryological origin, medullary thyroid carcinomas are exceptionally accompanied by a tumor derived from the follicular cells; this is defined as mixed medullary and follicular cell carcinoma. There have been controversies regarding the origin of this rare mixed thyroid carcinoma questioning whether or not a mixed carcinoma originates from a common cancer stem cell. We present a case of mixed medullary and follicular cell carcinoma in which two thyroid carcinomas were found intermingled in the thyroid as well as in the metastatic cervical lymph nodes. We examined the tumor by immunostaining with thyroglobulin, calcitonin, and thyroid transcription factor-1, and also reviewed the literature and discuss the origin of this rare mixed thyroid carcinoma.