The invention is a prosthetic device for improving function of a mitral valve. The device includes an adjustable and axisymmetrical-or-elongated sealing member configured for positioning between mitral valve leaflets. The device also includes an expandable anchor frame configured to be positioned within one or more heart chambers, for maintaining the sealing member at a desired position and size/shape between valve leaflets. The sealing member reduces mitral regurgitation be filling the gap that can occur between opposing leaflets of a damaged mitral valve, thus restoring proper mitral valve closure.