Background: Mandibular third molar impaction in adult patients is one of the most common problems in dental practice. However, limited data are available on the association between mandibular third molar impaction and craniofacial skeletal problems. Aim: This study aimed to establish a prediction model to evaluate the risk factors for mandibular third molar impaction. Methods: Panoramic and lateral cephalometric radiographs of orthodontic patients aged ≥20 years were obtained from the Tri Service General Hospital. The radiographs were analyzed by a single examiner. Panoramic radiographs were used to evaluate the mandibular third molar impaction. Lateral cephalometric radiographs were used to evaluate five linear and six angular measurements to define the facial pattern of the participants. The association between cephalometric measurements and mandibular third molar impaction was determined using one way analysis of variance and logistic regression analysis. Results: This study included 60 participants. The incidence of mandibular third molar impaction was associated with the mandibular plane angle (MPA) and mandibular length (Co Gn) (P < 0.001). The presence of mandibular third molar impaction was found to gradually increase with increasing MPA and decreasing mandibular length. Conclusion: Individuals with a greater MPA and lesser mandibular length had a higher incidence of mandibular third molar impaction.