A nineteen-year-old Warmblood mare presented to the Equine and Nemo Farm Animal Hospital with atrial fibrillation of at least three years’ duration. Cardiac auscultation revealed tachycardia, an irregularly irregular rhythm, and a grade III/VI heart murmur. Atrial fibrillation was confirmed on an electrocardiogram. Mild structural changes were identified on echocardiogram, cardiac troponin was normal, and bloodwork was unremarkable. A diagnosis of lone atrial fibrillation was made and transvenous electrical cardioversion (TVEC) was elected to convert the mare back to sinus rhythm. One transvenous electrode was placed in the left pulmonary artery and another in the right atrium using ultrasound and radiographic guidance. Under general anesthesia, a series of electrical shocks were delivered at increasing energies until cardioversion was achieved at 200 J. The patient recovered uneventfully from anesthesia, did not experience any clinically significant procedural complications, and was in sinus rhythm at the time of discharge. Prognosis for return to performance following TVEC is excellent, though there is risk of spontaneous reversion to atrial fibrillation.