A 24-year-old female with a diagnosis of primary carnitine deficiency, a rare inherited metabolic disorder predominantly described in the paediatric literature that causes cardiomyopathy, presented for evaluation after three months of nonadherence with prescribed carnitine therapy. Initial echocardiography demonstrated severe left ventricular dilation (104 ml/m2) (normal < 76 ml/m2) with moderate systolic dysfunction (ejection fraction 40%) and severe right ventricular dilation with mild systolic dysfunction. Carnitine replacement was commenced, and a cardiac magnetic resonance imaging (MRI) performed five days later demonstrated dramatic improvement in biventricular function with normalization of left and right ventricular systolic function. To our knowledge, this is only the second case describing the rapid reversal of cardiomyopathy in an adult patient with this rare condition.