To investigate the effect of mesioangular impaction of the mandibular third molars on distal caries and distal bone loss (DBL) in the adjacent mandibular second molars to provide a basis for prophylactic impacted third molar extraction. We retrospectively analyzed panoramic radiographs of 736 patients with mesioangularly impacted mandibular third molars at the Department of Stomatology, Shanghai Fifth People's Hospital. The chi-square test, the Kruskal-Wallis test, and logistic regression were performed to analyze the relationship of patient age, sex, mesial inclination angle, distal caries, and DBL in the mandibular second molars. Patient aged over 30 years and a mesial inclination angle > 30° were risk factors for distal caries in the mandibular second molars (p < 0.05). Patients aged over 20 years and a mesial inclination angle of 60°-90° were risk factors for DBL (p < 0.05). In patients with mesioangularly impacted mandibular third molars, age and the mesial inclination angle were positively correlated with the severity of distal caries and DBL in the adjacent mandibular second molars. These findings could guide dental practitioners to prevent complications of the adjacent molars during early prophylactic extraction of the mandibular third molars with mesioangular inclination.