Mediastinal germ cell tumor (MGCT), which accounts for 1% to 3% of extragonadal germ cell tumors, has unique manifestations; it is associated with several types of hematologic malignancy, particularly myeloid neoplasm. The aim of this study was to report the 10-year incidence, clinical characteristics, and outcomes of MGCT at Thailand’s national pediatric tertiary referral center. This retrospective study included patients diagnosed with MGCT at the Department of Pediatrics, Siriraj Hospital during 2005 to 2014. Eight patients (all male) were diagnosed with MGCT. Five of 8 patients were found to have hematologic abnormalities. Three patients were diagnosed with acute myeloid leukemia (AML) (one patient with M1, another having M7, and the other with M0). Another patient had mixed MGCT with mediastinal myeloid sarcoma (MMS). The other patient had malignancy-associated hemophagocytic lymphohistiocytosis syndrome (M-HLH). Isochromosome 12p was detected in 3 patients (AML , mixed MGCT/MMS ). Four of 5 patients with hematologic abnormalities died of hematologic abnormalities or treatment complication (AML , M-HLH ). One patient with mixed MGCT/MMS survived with chemotherapy. All patients with AML and MMS were nonseminomatous MGCT and the onset of myeloid malignancies were within 1 year after the diagnosis of MGCT. Associated hematologic malignancies should be suspected in MGCT with abnormal blood count or hematologic symptoms. Isochromosome 12p was the most common cytogenetic finding in MGCT-associated myeloid malignancies patients. Those with nonseminomatous MGCT should have their blood count carefully monitored especially during the first year after the diagnosis of MGCT. Better treatment alternatives for MGCT with associated hematologic malignancies are warranted to ameliorate adverse outcomes.