A 15-year-old girl presented with diabetes mellitus without ketoacidosis, initially treated as type 1 diabetes (T1DM). Despite a family history of diabetes, the patient showed reduced insulin requirements (0.3 U/kg/day) and tested negative for autoantibodies associated with T1DM. Following ISPAD guidelines, genetic testing was performed, revealing a heterozygous mutation in the HNF4α gene, associated with MODY 1 (Maturity-Onset Diabetes of the Young). The patient maintained good glycemic control with insulin therapy, but the case raises questions about the timing and approach to transitioning to sulfonylureas, the first-line treatment for MODY 1, which could potentially improve glycemic control and the quality of life.