The Na-Ca exchanger (NCX1) is the dominant Ca extrusion mechanism in cardiac myocytes. NCX1 activity is inhibited by intracellular Na via a process known as Na-dependent inactivation. A central question is whether this inactivation plays a physiological role in heart function. Using CRISPR/Cas9, we inserted the K229Q mutation in the gene (Slc8a1) encoding for NCX1. This mutation removes the Na-dependent inactivation while preserving transport properties and other allosteric regulations. NCX1 mRNA levels, protein expression, and protein localization are unchanged in K229Q male mice. However, they exhibit reduced left ventricular ejection fraction and fractional shortening, while displaying a prolonged QT interval. K229Q ventricular myocytes show enhanced NCX1 activity, resulting in action potential prolongation, higher incidence of aberrant action potentials, a faster decline of Ca transients, and depressed cell shortening. The results demonstrate that NCX1 Na-dependent inactivation plays an essential role in heart function by affecting both cardiac excitability and contractility.