This article presents the report of a case of mandibular osteoma and reviews the characteristics, diagnosis, and treatment. A 6-year-old female spayed cat was referred for an evaluation of a mandibular mass. A non-painful swelling of the left mandible was incidentally noted three weeks ago during a health examination and dental scaling. Further examinations were conducted, radiography revealed a mass in the left caudal mandible extending to the ramus with bone opacity, and computed tomography (CT) scans demonstrated a hyperostotic lesion centered on the same area. The initial histopathological diagnosis of the mandibular mass biopsy was osteoma or reactive bone. Complete surgical excision is a curative treatment for osteoma, while debulking is another surgical option. According to one case in a previous report, this case underwent a debulking surgery and recurred after nine months. Therefore, we opted for a caudal mandibulectomy for our patient. The excised portion of the caudal mandible, including the mass, was sent to the histopathologist again, and the final diagnosis of endosteal osteoma was made. Post-surgery, the patient is doing well, with a good quality of life and no gross signs of recurrence and complications to date.