Approximately 6% of patients with primary mediastinal nonseminomatous germ cell tumors (MNSGCTs) develop hematologic malignancies associated with germ cell tumors (HMGCTs), resulting in a dismal prognosis. We report two patients who developed HMGCTs after complete remission of MNSGCTs. They underwent four cycles of chemotherapy and surgical resection of the residual mediastinal mass, resulting in complete remission of the MNSGCTs. Patient 1 (case 1) was a 26-year-old man who developed myelodysplastic syndrome with thrombocytopenia nine months after the diagnosis of MNSGCTs. Despite treatment with chemotherapy and allogeneic hematopoietic stem cell transplantation, he died 24 months after the onset of MNSGCTs. Patient 2 (case 2) was a 16-year-old man who developed chronic myelomonocytic leukemia 9 months after MNSGCT diagnosis. Despite treatment with chemotherapy, he died 14 months after the onset of MNSGCTs. For early detection of HMGCTs, bone marrow examination and chromosomal analysis should be performed at the time of MNSGCT diagnosis, and hematological examination should be carefully monitored, especially for the first year after the diagnosis.