Background: This study evaluated the results of intramedullary osteosynthesis with titanium elastic nail (TEN) in the surgical treatment of Monteggia lesions in children aged 12 and under. Methods: Patients who underwent surgery with the TEN method between 2013 and 2019 were screened retrospectively. The inclusion criteria were patients who failed conservative treatment, a history of acute trauma (< 14 d after injury), age 12 years and under, patients who underwent intramedullary osteosynthesis with TEN when surgical treatment is required, a follow-up period of over 12 months, absence of congenital upper extremity and bone anomalies, and no history of trauma to the ipsilateral elbow. For functional classification, the Grace and Eversmann Criteria were used to evaluate union, pronation, and supination, while the Mayo Elbow Performance Score was used to assess overall elbow function and limitations. Results: Thirty-eight patients (26 male/12 female) were included in the study. The median age was 8.0 (6 to 10) years old and follow-up period was 26.0 (16 to 39) months. The mean value of the Mayo Elbow Performance Score was 97.1, 32 cases were excellent and 6 cases had good results. While the average age was 9.4 years in patients with mobility limitations, it was 7.1 years in patients without mobility limitations. Statistically, less movement restriction was observed in patients of younger age. Conclusion: Length unstable ulnar fracture pattern for Monteggia lesions and those with secondary loss of reduction, intramedullary stabilization of the ulna fracture with TEN helps to achieve good functional and radiologic outcomes.