Sarcomatoid carcinoma (SC) is a rare lung cancer subtype with poor prognosis and lack of effective treatment regimens. Studies concerning SC indicated common programmed death ligand-1 (PD-L1) overexpression and higher tumor mutational burden, leading to potential benefits from immunotherapy. The present case is the first report employing PD-L1 inhibitor durvalumab following definitive concurrent chemoradiotherapy (cCRT) in a patient with mediastinal lymph node metastatic SC, which was considered as a high probability of pulmonary origin but unclear primary lesion. After the 19-month follow-up, there was neither local recurrence nor distant metastasis. The patient was in a good condition, with the thoracic lesion controlled at Partial response-Response Evaluation Criteria in Solid Tumors (PR-RECIST). Except for grade 2 esophagitis, none of the other adverse events was observed. Our first attempt to adopt the consolidation immunotherapy after cCRT in unresectable locally advanced mediastinal SC exhibited improved local control, manageable safety, and potential survival benefits, representing a novel and promising therapeutic option for SC and encouraging further research exploration of this regimen in the future.