Background: Diabetes mellitus is a chronic disease associated with multiple organ complications and an increased risk of hospital readmission. Hospital discharge planning is essential for coordinating continuity of treatment, self-management, and family and social support. However, evidence regarding the implementation of discharge planning for patients with diabetes and its associated challenges in Mongolia remains limited. Aim: To identify the implementation, process, and challenges associated with discharge planning for patients with diabetes and multi-organ complications at the Mongolian-Japan hospital of the MNUMS. Materials and Methods: A qualitative case study was conducted at the Mongolia–Japan Hospital of MNUMS, focusing on its hospital discharge planning service. Purposive sampling was used to recruit seven healthcare professionals: a health social worker, a dietitian, two endocrinologists, a diabetes specialist nurse, a clinical pharmacist, and a deputy director responsible for clinical affairs. Data were collected through semi-structured individual interviews and document analysis of the hospital’s internal discharge planning procedure and three multidisciplinary team meeting protocols. Interview data were analyzed using Braun and Clarke’s six-phase thematic analysis approach. Findings from the three data sources were compared using a thematic matrix. Ethical approval was obtained from the MNUMS Ethics Review Committee (Decision No. 25-26/01-02, September 2025). Results: Five major themes were identified: (1) risk-based multidisciplinary collaboration; (2) gaps in legal, policy, and standardization frameworks; (3) financing and health insurance constraints; (4) limitations in human resources and professional capacity; and (5) weaknesses in post-discharge follow-up and family support. Risk assessment and multidisciplinary case discussions for high-risk patients were documented; however, meetings were irregular, and follow-up on decisions was inconsistent. The absence of unified regulations, inter-institutional information exchange, and feedback mechanisms hindered continuity of care. Financial barriers to consultations, care services, medicines, and medical supplies limited patients’ ability to follow recommendations. Workload and shortages of specialized personnel further constrained implementation. Post-discharge follow-up lacked standardized timelines, frequency, and case-closure criteria. Comparison of 15 subthemes across the three data sources identified six areas of convergence, eight complementary findings, and one divergence. The meeting protocols concerned patients without diabetes and were therefore used as supplementary evidence of general institutional practice. Conclusion Although core discharge planning processes and multidisciplinary collaboration have been established at the Mongolia–Japan Hospital, gaps remain in regulatory frameworks, financing, human resources, post-discharge follow-up, and inter-institutional coordination. Discharge planning should be strengthened as a continuous process integrating post-discharge care and family support.