Primary mediastinal nonseminomatous germ cell tumors (NSGCT) have a worse prognosis than gonadal germ cell tumors (GCTs). Malignant transformation of teratomatous components of GCT to a somatic malignancy is rare. We present a case of primary mediastinal NSGCT with malignant transformation of a teratoma in an active duty airman who presented with a 12-cm anterior mediastinal mass and vertebral body lesion. Pathology confirmed NSGCT with yolk sac and teratomatous components. Chemotherapy for GCT normalized serum tumor markers with little effect on the mediastinal mass. Incomplete resection of the residual tumor revealed high-grade mixed sarcoma. Serum tumor markers rose 3 months following resection with metastatic disease to bone and liver. Chemotherapy targeting sarcoma induced a partial response, but the patient developed acute myeloid leukemia refractory to chemotherapy. We discuss our approach to management of this complicated patient.