Congenital hyperinsulinism (HI) is a disorder of insulin regulation that results in hypoketotic hypoglycemia and is associated with a high risk of neurological sequalae from hypoglycemia-induced brain damage. There is only 1 Food and Drug Administration-approved drug for the treatment of HI, and up to 60% of patients do not respond to it; thus, there is a significant unmet need for new treatments. We report a case of a 29-year-old female with HI due to an activating glucokinase pathogenic variant, who was diagnosed as an adult and continued to have persistent hypoglycemia despite standard-of-care therapy. The patient was enrolled in the phase II clinical trial evaluating efpegerglucagon, a novel long-acting glucagon analog. She has continued treatment with efpegerglucagon through a single-patient Investigational New Drug Expanded Access Program and has demonstrated improved glycemic control throughout 10 months of treatment. Efpegerglucagon may be a viable treatment option or adjunct therapy for patients with HI who are unresponsive or partially responsive to currently available therapies.