A spayed female Miniature dachshund presented with a large mediastinal mass. During thymectomy, the left phrenic nerve, which was encircled by the neoplastic tissue, was resected en bloc with the tumour and the right phrenic nerve was damaged by thermal coagulation from electrosurgery. Postoperatively, the dog exhibited significant respiratory distress and paradoxical respiratory movement. Bilateral diaphragmatic paralysis was confirmed on inspiratory radiographs, and lack of structural abnormalities on thoracic computed tomography was noted. The dog collapsed 5 days postoperatively from respiratory fatigue. Thus, salvage surgery and tensioning diaphragmoplasty were performed. Briefly, The diaphragm was partially resected, primarily closed and reinforced with a polypropylene mesh sutured on the abdominal surface. Thereafter, the dog's condition improved significantly. The dog did not present with respiratory issues until she died of urothelial carcinoma on day 375. In conclusion, this surgical technique can be considered to re-establish the function of the diaphragm.