Introduction: Improper disposal of unused and expired medicines poses severe threats to environmental safety and public health, notably accelerating antimicrobial resistance (AMR). Globally, pharmaceutical consumption rose by 14% between 2019 and 2023. Despite this surge, Mongolia continuously lacks an established national pharmaceutical take-back framework, and public behaviors regarding residential medical waste management remain insufficiently evaluated. This study aimed to assess the knowledge, attitudes, and practices of adult residents in Ulaanbaatar concerning medication disposal. Methods: A quantitative cross-sectional survey was conducted between January and March 2026 among 301 adult citizens across nine districts of Ulaanbaatar. Data were collected via a structured questionnaire evaluating demographic parameters, knowledge (10 items), attitudes (10 items), and practices (8 items). Statistical analysis was executed using IBM SPSS v26.0, applying descriptive frequencies, percentages, and one-way ANOVA. Conclusion The findings reveal a prominent knowledge-behavior gap among citizens. While the mean knowledge score was moderate (62.4%), with 92.2% recognizing the dangers of expired drugs, practical behaviors were profoundly unsafe. Specifically, 86.4% of participants discarded medication alongside ordinary household waste, 94.0% had never returned products to pharmacies, and 0% had utilized a designated medical disposal point. Conversely, an overwhelmingly positive public attitude was observed, as 88.4% expressed willingness to return unused medicines if a formal pharmacy-based collection system existed. This critical disparity underscores that public awareness campaigns alone are inadequate. There is an urgent necessity to integrate pharmacy-led take-back programs into national environmental and health policies, establish accessible urban disposal infrastructure, and enforce rigorous pharmaceutical waste standards in Mongolia.