Background: Mandibular osteomyelitis is commonly associated with invasive surgery and sequestrectomy. Here, we report a unique case in which it was possible to choose minimally invasive treatment during the exacerbation of osteomyelitis at the stage of sequestrectomy owing to the use of and correct assessment with ultrasonography (USG). This report aimed to present wide possibilities of USG for accurate diagnosis and minimally invasive management of chronic osteomyelitis. Case Presentation: A 50-year-old woman presented with significant swelling in the left masseteric region, trismus, and severe pain in the area of a previously extracted lower third molar. The imaging protocol included panoramic radiography, USG, and cone-beam computed tomography. Using USG, it was possible to identify the intermediate stage of abscess/phlegmon formation in the masseteric area, which would require an extraoral incision to drain the purulent focus. Conclusion: USG allows oral and maxillofacial surgeons to obtain a precise understanding of the condition of the tissues (e.g., bone surface, masseter muscle, subcutaneous tissue) and pathologic changes (e.g., periosteal reaction, bony defects) due to purulent processes in the area of the lateral mandibular ramus surface and surrounding soft tissues. The presence of purulent material and its motion on sonopalpation can be clearly identified using USG. Therefore, it is possible to plan for a less invasive surgical strategy.