Visceral injury, pneumomediastinum, is a clinical manifestation that is hardly attributed secondary to facial trauma. The correlation between post-oral maxillofacial fracture mediastinal and cervical emphysema manifestations is rarely documented in the literature. This case report describes an atypical case of pneumomediastinum consequent to isolated oral-maxillofacial trauma. The presented patient suffered from a fracture of the ramus of the mandible from the angle to the coronoid process without displacement and a fracture of the posterior wall of the left maxillary sinus with small invagination to the lumen of the sinus. The entrance route can be hypothesized through the fracture of the posterior wall of the left maxillary sinus. This case reinforces the importance of correct diagnosis and proper treatment modalities for similar oral-maxillofacial injuries.