Lower lip reconstruction in cases with a full-thickness defect over one-third of the vermilion is challenging. Numerous conventional techniques have been applied with unsatisfactory surgical outcomes because of microstomia and oral commissure blunting due to shortened horizontal lip length. Herein, we present a case in which a full-thickness lower lip defect of more than one-third of the horizontal lip length was covered with a novel mucosal roofing flap reconstruction to minimize the loss of horizontal lip length and to preserve mouth opening. No recurrences or metastases were observed during 3 years and 6 months of follow-up, with horizontal lower lip length maintained and mouth opening of 2.5 finger breadths.