Introduction: Systematic analysis of adverse drug reactions (ADRs) is essential for enhancing drug safety policies in Mongolia. This study analyzed ADR patterns and reporting trends between 2020 and 2025. Methods: This study employed a retrospective cross-sectional design using ADR reports recorded between 2020 and 2025. The dataset included variables such as drug name, drug classification, clinical manifestations of ADRs, and affected organ systems. Data were entered into Microsoft Excel and analyzed using SPSS version 25. Drugs were classified according to the World Health Organization (WHO) Anatomical Therapeutic Chemical (ATC) classification system, and ADRs were analyzed at the organ system level. Conclusion Analysis of 634 reported adverse drug reactions (ADRs) in Mongolia from 2020 to 2025 showed that antibiotics, particularly third-generation cephalosporins, accounted for the majority of ADRs. According to Hartwig’s severity assessment, 70–80% of ADRs were mild, 15–20% moderate, and 5% severe, indicating that while most reactions did not require intervention, a small proportion could be lifethreatening. These findings highlight the need to strengthen pharmacovigilance systems, improve ADR reporting, and enhance the quality of data in Mongolia.