Pyoderma gangrenosum (PG) is a rare, neutrophilic dermatosis which is characterized by painful, necrotic ulcer with violaceous border that heals with cribriform scar. Although the etiopathogenesis of PG is not known exactly, it can be triggered by many factors such as genetics, autoimmune, pathergy phenomenon, drugs, and paraneoplastic. It is frequently associated with autoimmune pathogenesis such as inflammatory bowel disease and rheumatologic disease. It can also be associated with hematological or solid organ malignancies, and then, it is called paraneoplastic PG. The association of PG with myasthenia gravis and thymoma has not been previously reported. In our case, these three diseases with a common paraneoplastic pathogenesis were seen together and the coexistence of the three diseases is rare. Treatment of PG should be decided according to the severity, spread of the lesions, concomitant disease, medical condition, and tolerance of the patient. The purpose of treatment is to control the lesions and related diseases for a long time with minimal side effects. Mycophenolate mofetil treatment was used safely and successfully for both generalized MG and PG in our case.