Graves' disease is a common cause of hyperthyroidism that can lead to multiple cardiovascular complications. Herein is described the case of a 44-year-old male who presented with new-onset atrial fibrillation and mitral regurgitation secondary to flail anterior mitral leaflet with chordae tendineae rupture. This is a rare complication for Graves' disease, and has been reported only twice previously. It was hypothesized that this complication is secondary to Graves'-associated myxomatous degeneration of the mitral valve in the presence of a hyperdynamic circulation.