Prosthetic aortic valve and conduit dehiscence with periconduit cavity and ascending aortic aneurysm is an uncommon complication of aortic root surgery. It is usually recognizable at echocardiography due to an abnormal position of the prosthetic valve and conduit in relation to the native aortic annulus in conjunction with an abnormal echolucent periconduit space that fills with color flow. Mitral regurgitation is an unusual complication of this condition. We present a patient with severe mitral regurgitation secondary to prosthetic aortic valve and conduit dehiscence with a large periconduit cavity and aneurysm of the intervalvular fibrosa. The mechanism of mitral regurgitation is secondary to functional involvement of the anterior mitral valve leaflet and intervalvular fibrosa with anterior mitral leaflet restriction in conjunction with mild left ventricular remodeling. Significant mitral regurgitation persisted post resection of the periconduit cavity and aortic valve replacement, requiring mitral valve replacement. This case study reports a new mechanism of mitral regurgitation in the setting of prosthetic aortic valve and conduit dehiscence.