Background: The incidence of secondary somatic malignancy (SSM) arising from teratoma is increased in 2 settings; late relapse and primary mediastinal nonseminomatous germ cell tumors (PM-NSGCT). Here, we report the clinical features and outcomes of patients with SSM in the setting of PM-NSGCT. Methods: Between 1985 and 2018, 29 patients with PM-NSGCT and SSM who had sufficient clinical follow-up to evaluate outcome were identified. Clinical and pathologic parameters were reviewed. The Kaplan-Meier method was used to estimate overall survival (OS) from time of SSM diagnosis and the log rank test to compare estimates. Results: Median age was 28 years (range 18-59) and all patients were male. Most presented with local symptoms (n=24, 83%), elevated tumor markers (n=26, 90%), and disease isolated to the mediastinum (n=25, 86%). A total of 39 SSM histologies were present in the 29 cases, with 8 (28%) having 2 (n=6) or 3 (n=2) SSM histologies; 25 (86%) also had viable non-teratomatous GCT in the mediastinal mass. Sarcoma was found in all 29 cases including rhabdomyosarcoma (n=15), angiosarcoma (n=6), sarcoma NOS (n=5), spindle cell (n=4), PNET (n=3), and other (n=3). Non-sarcoma histologies (n=1 each) included AML, SCC, and neuroblastoma. Most patients received GCT-directed chemotherapy followed by an attempt at surgical resection (90%). With a median follow-up of 2 years for survivors, median OS was 1.8 years (95% CI 0-3.9 years), with 18 patients succumbing to disease. Complete surgical resection was achieved in 23 men (79%) and was associated with superior OS (3.1 vs. 0.3 years, p=0.005). At relapse or progression, 11 received SSM histology-directed and 7 GCT-directed chemotherapy with no difference in OS (1.3 vs. 1.2 years, p=0.993). 7 patients developed SSM in the form of leukemia, a finding associated with significantly inferior OS (0.3 vs. 3.0 years, p=0.009). Conclusions: Sarcoma is the predominant SSM histology associated with PM-NSGCT and portends a poor prognosis even with initially localized disease. Complete resection following chemotherapy is critical to achieving long-term survival whereas SSM in the form of leukemia portends especially poor outcome.