Isolated mitral insufficiency requiring surgical correction is extremely rare in Williams syndrome (WS). We report the case of a 25-year-old man with WS, who suffered congestive heart failure caused by severe mitral insufficiency and atrial fibrillation. We found extensive anterior leaflet prolapse, but no other cardiovascular disorders such as supra-aortic or pulmonary arterial stenosis. He underwent successful radical valve repair with a concomitant Cox-maze procedure. This reconstructive procedure was appropriate and effective surgical treatment for isolated mitral valve disease associated with WS.