Background/Objectives: Trauma from occlusion (TFO) is a modifying factor in periodontal disease progression, yet its morphometric impact on mandibular molars remains underexplored. The crown–root ratio (CRR), traditionally used in prosthodontic prognosis, may also serve as a diagnostic marker for structural changes in periodontally compromised teeth. This study evaluated the relationship between crown and root dimensions and clinical/radiographic parameters in mandibular first molars with TFO and developed predictive models emphasizing the role of CRR. Methods: This retrospective cross-sectional study included 99 periodontitis patients clinically and radiographically diagnosed with TFO. Digitized periapical radiographs of mandibular first molars (tooth 36 or 46) were analyzed to measure clinical and radiographic CRR, crown and root length, tooth inclination, alveolar bone loss, and root morphology. Correlation and stepwise multiple regression analyses identified predictors of crown and root length. Results: Males had significantly greater crown length (7.6 vs. 7.2 mm), root length (13.3 vs. 12.3 mm), and radiographic CRR (1.2 vs. 1.0) (p = 0.008). Clinical CRR showed a moderate positive correlation with crown length (r = 0.526) and a strong inverse correlation with root length (r = −0.735) (p < 0.001). Regression models revealed that clinical CRR, root length, and sex significantly predicted crown length (R2 = 0.955), while CRR and crown length predicted root length (R2 = 0.958). Conclusions: This study demonstrated that the clinical crown–root ratio (CRR) is a strong predictor of both crown and root lengths in mandibular first molars affected by trauma from occlusion (TFO) in periodontitis patients. © 2025 by the authors.