There are nine published reports of trileaflet mitral valves globally. As such their implication on health outcomes and associations with other disease is uncertain. This case describes a 62-year-old man presenting with exertional dyspnoea and hypertension. It describes an early misdiagnosis of hypertrophic cardiomyopathy and highlights that clinicians should be alerted in cases of very high left ventricular outflow gradients in the presence of eccentric mitral regurgitation (MR). Here the MR was caused by a rare congenital deformity whereby a deep cleft in the posterior leaflet resulted in a tricuspid appearance. We present the natural disease course of a trileaflet mitral valve and without intervention over 13 years from symptom onset to the development of severe MR.