Introduction: Mandibular invasion is an adverse prognostic factor in oral squamous cell carcinoma. However, the impact of positive bone margins on survival remains unclear. This study evaluates survival outcomes in patients with mandibular invasion and compares outcomes based on bone margin status. Methods: Data of 214 patients undergoing segmental mandibulectomy (2021–2024) were analyzed. After excluding patients without mandibular invasion, 160 patients were included. Disease-free survival (DFS) and overall survival (OS) were assessed using Kaplan–Meier and Cox regression analysis. Results: At a mean follow-up of 23.2 months, mean DFS and OS were 21 and 24 months, respectively. Seven patients had positive bone margins despite negative intraoperative cytology. These patients had significantly worse DFS (11 vs 21 months, p < 0.001) and OS (18.8 vs 38.7 months, p = 0.008). Positive bone margin was an independent predictor of poorer survival. Conclusion: Positive bone margins significantly worsen survival, and adjuvant CTRT does not mitigate this effect. Revision surgery should be considered despite higher morbidity.