Ranger, a five year old male neutered Domestic Shorthair cat was presented to the Cornell University Hospital for Animals (CUHA) for evaluation of lethargy, weakness, and inappetance. Additional historical findings included dysphonia, dysphagia, and regurgitation. A physical examination disclosed diffuse neuromuscular weakness and seborrhea sicca. Intravenous edrophonium was administered, and a marked increase in neuromuscular strength was noted. An acetylcholine receptor antibody titer confirmed a diagnosis of myasthenia gravis. Thoracic radiography, thoracic ultrasound, and computed tomography disclosed a cranial mediastinal mass. A median sternotomy was performed and the mass was excised, with histopathology yielding a diagnosis of thymoma. Pyridostigmine was administered for 2.5 months, and Ranger maintained a significant improvement in neuromuscular strength. However, at serial re-evaluations, exfoliative dermatitis, pemphigus dermatitis, and pleural effusion were documented. The exfoliative dermatitis and pemphigus dermatitis completely resolved and the pleural fluid partially resolved in response to discontinuation of pyridostigmine administration and initiation of prednisolone administration. The pathophysiology and treatment of thymoma and paraneoplastic myasthenia gravis and the associated complications are discussed in the following presentation.