Background: The most common cranial mediastinal masses affecting dogs and cats include lymphoma and thymic epithelial tumours (TETs). In this report, a cat with a cranial mediastinal mass was diagnosed with a thymic carcinoma subtype squamous cell carcinoma, which has rarely been reported in cats. Management of this subtype with a combination of surgery and chemotherapy has so far not been reported. This case report describes the treatment with surgical management followed by intracavitary carboplatin. Case Description: A 12-year-old male neutered domestic short hair cat was referred for lethargy, hyporexia and weight loss, and was diagnosed with a cranial mediastinal mass radiographically. Initial cytology through fine needle aspirates were suggestive of carcinoma. Computed tomography was performed which did not show evidence of other primary tumour source or metastasis, and hence surgical resection was recommended. Intraoperative findings revealed local invasion of the surrounding tissues, including major vasculature and nerves, although histopathological assessment showed no local lymph node involvement. Intracavitary carboplatin chemotherapy was administered two weeks post surgery. The patient was humanely euthanased four weeks post surgery due to evidence of local recurrence causing significant respiratory compromise. Conclusion: A combination of surgical excision as well as intracavitary carboplatin does not seem to be effective for the treatment of this thymic carcinoma subtype, with evidence of early recurrence and return of clinical signs.