Objective Metastatic lesions account for 1% of all oral and maxillofacial malignancies. A quarter of gnathic metastases are discovered before the primary tumor is known. We present a case of adenocarcinoma of the mandible, as first evidence of advanced lung cancer. Findings A 65-year-old male presented to the oral surgeon with a 6-month history of lower left jaw pain. Panoramic radiograph showed an ill-defined radiolucency inferior to the mandibular canal. A biopsy revealed a scattered glandular proliferation, with a few areas consisting of cribriform architecture and foci of back to back glandular lumens. No features of mucoepidermoid carcinoma were identified. A subsequent PET CT scan showed an ill-defined nodule in the left upper lobe of the lung measuring up to 2.5 cm in greatest dimension. Multiple hilar, subcarinal, and paratracheal nodules were also identified, concerning for nodal metastasis. Immunohistochemical stains were then performed on the original biopsy from the mandible and the tumor cells stained positive for TTF1, Cytokeratin 7, and Napsin A, suggestive of adenocarcinoma of pulmonary origin. Consequently, MRI of the brain identified lesions in the parietal and frontal lobes, measuring up to 3.4 cm. Treatment for the patient included chemotherapy with Pemetrexed (Alimta) and carboplatin, immunotherapy with Keytruda, and once tapered off, palliative radiotherapy. Conclusion Primary adenocarcinoma of the jaw is extremely rare, except for 2–3% central mucoepidermoid carcinomas. The possibility of a metastatic tumor should be a consideration when encountering unusual histomorphology of an adenocarcinoma in the jaw bone.