An incidental large mediastinal mass on the left hemithorax was noted in the chest radiograph of a 21-year-old male with a history of active smoking (five pack years) after a manual reduction for left shoulder dislocation. Horner syndrome (HS) was elicited from history and physical examination. A contrast computed tomography (CT) of the thorax showed a large (8.1 x 7.3 x 7.3 cm), well-defined, heterogeneous enhancing mass over the left apical-posterior mediastinum. The CT-guided biopsy showed spindle cells, suggestive of lung neoplasm. Initial concerns were thus present for lung malignancy and he was referred to thoracic surgery for further evaluation. The thoracic team decided on a left video-assisted thoracoscopic surgery (VATS) and the tumor was resected uneventfully. Unfortunately, the Horner syndrome persisted. The final histopathology confirmed schwannoma. He was briefed regarding the benign prognosis and was eventually discharged. This study serves to illustrate the incidental finding of schwannoma, a rare diagnosis, and its association with Horner syndrome.