Supraventricular tachycardia is the most common tachyarrhythmia in pediatrics. Although postoperative junctional ectopic tachycardia (JET) is a known complication of congenital heart surgery that is typically transient, congenital JET is rare and requires aggressive treatment to maintain hemodynamic stability. We describe the case of a 3-month-old, previously healthy female who presented with heart failure and cardiogenic shock secondary to congenital JET for whom extracorporeal membrane oxygenation (ECMO) provided time for selection of effective therapy. Adenosine, cardioversion, and transesophageal pacing were unsuccessful, and her echocardiogram demonstrated bilateral atrial dilation and severe left ventricular systolic dysfunction. Approximately 8 hours after presentation, venous-arterial ECMO was commenced allowing for successful treatment with amiodarone. Her electrocardiogram demonstrated atrioventricular dissociation consistent with JET. She was successfully decannulated from ECMO after 6 days. Her discharge echocardiogram showed normal ventricular function, and she had no significant ECMO sequelae. This case demonstrates the value of early ECMO initiation for cardiovascular support in pediatric patients with a life-threatening arrhythmia and in cardiogenic shock. ECMO support can allow for full diagnostic and therapeutic decisions to effectively reverse the consequences of uncontrolled arrhythmias unrelated to surgical complications.