Mixed germ cell tumor (MGCT) mainly occurs in young women's ovaries and men's testicles and rarely occurs outside the gonad. Fewer than 10 cases of mediastinal MGCT are available in PubMed, Embase, and other databases in English, while mediastinal MGCT with three pathological components, such as yolk sac tumor, immature teratoma, and embryonal carcinoma, has not been reported previously. A 12-year-old male sought medical attention for chest discomfort and underwent a computed tomography (CT) scan. A large soft tissue mass occupying most of the left thoracic cavity and mediastinum was detected. A CT-guided biopsy was performed, and an MGCT was diagnosed with pathological components, including yolk sac tumor, immature teratoma, and a small amount of embryonal carcinoma. Due to the large size of the tumor, the patient was treated with an EP regimen (etoposide + cisplatin) and paclitaxel + ifosfamide + cisplatin interstitial chemotherapy. The patient was followed up for 6 months and was alive with the disease. To the best of our knowledge, this is the 10th patient with MGCT in the mediastinum. The incidence of mediastinal MGCT is low, but it should still be considered one of the differential diagnoses of isolated pleural fibroma and neurogenic tumors.