Background: Free fibular flap (FFF) is the main modality in the reconstruction of maxillofacial defects. Micrognathia is a condition where the mandible is smaller than it should be. Micrognathia is typically treated with mandibular distraction osteogenesis, bilateral sagittal split osteotomy, and conventional costochondral graft. In cases in which these procedures cannot be performed, FFF becomes a suitable alternative. Publications regarding the use of FFF in cases of micrognathia are currently still limited. In this case, we present the use of FFF in the reconstruction of a severely micrognathic mandible in an adult patient. Case Presentation: A 28-year-old woman with micrognathia was referred to Dr. Ciptomangunkusumo Hospital for reconstruction. The patient had previously undergone temporomandibular joint (TMJ) reconstruction surgery with interpositional TMJ arthroplasty 10 years prior due to an ankylosing TMJ that prevented the opening of the jaw. The treatment for ankylosing TMJ was necessitated due to the patient's inability to open their mouth, resulting in significant challenges with alimentation, speech, and facial morphology. Here the authors reported mandibular lengthening reconstruction in the anteroposterior axis with FFF. A surgical osteotomy was performed on the posterior segment of the bilateral mandibular ramus, then the segmented fibula was inserted into the resulting defect. Flap refinement was also performed following FFF. Postsurgical evaluation revealed notable enhancements in the patient's esthetic appearance, occlusal function, and amelioration of obstructive sleep apnea symptoms. Conclusions: FFF for mandibular reconstruction is feasible and effective in improving esthetic and functional outcomes in patients with micrognathia. © 2025 Lippincott Williams and Wilkins. All rights reserved.