Leiomyosarcoma is a malignant neoplasm of smooth muscle origin that manifests itself uncommonly in the oral cavity because of the paucity of smooth muscle in that location. To the best of our knowledge, only 10 cases of leiomyosarcoma primary to the jawbones have been reported in the English language literature. We report the first pediatric case of leiomyosarcoma arising from the mandible. Facial asymmetry and swelling were accompanied by a rapidly growing exophytic soft tissue mass that caused buccal displacement of the mandibular left permanent first molar. The lesion, observed radiographically as an extensive ill-defined area of osteolytic alveolar destruction, perforated the lingual cortex, displaced the inferior alveolar nerve canal inferiorly, and produced a "floating-in-air" appearance of the first molar. Diagnosis of leiomyosarcoma was made after initial incisional biopsy of the lesion. A 5-cm segmental mandibulectomy and supraomohyoid neck dissection were followed by reconstruction with a dynamic mandibular reconstruction plate and placement of a multidimensional mandibular distraction device in a transport rectangle of bone to promote bifocal distraction osteogenesis. Forty millimeters of distraction (the technical limit of the device) were performed; this was followed by terminal iliac crest bone grafting. Seventeen months after the definitive surgical procedure, the patient remains free of disease.