A self-forming mattress cover including a mattress gusset with a drawstring is provided. The gusset includes a sleeve attached to the gusset along a fold location. A drawstring is disposed within the sleeve. Ends of the drawstring are pulled into tension, thereby reducing the perimeter of the gusset at the fold location and drawing the gusset inward toward the center of the mattress. The gusset is formed to corner positions of the mattress by stretching and/or bunching of gusset material as tension is applied. The ends of the drawstring are secured to retain the tension state. A cushioned quilt top is attached to a free edge of the gusset. The self-forming mattress cover conforms to corner positions of a mattress without the need for mitering, ruffling, pleating, or sewing seams at the corner positions. The self-forming mattress cover also ensures proper alignment of the cushioned quilt with the mattress body.