Dilated cardiomyopathy (DCM) leads to heart failure, a leading cause of death in industrialized nations. Approximately 30% of DCM cases are genetic in origin, with some resulting from point mutations in cardiac myosin, the molecular motor of the heart. The effects of these mutations on myosin's molecular mechanics have not been determined. We have engineered two murine models characterizing the physiological, cellular, and molecular effects of DCM-causing missense mutations (S532P and F764L) in the α-cardiac myosin heavy chain and compared them with WT mice. Mutant mice developed morphological and functional characteristics of DCM consistent with the human phenotypes. Contractile function of isolated myocytes was depressed and preceded left ventricular dilation and reduced fractional shortening. In an in vitro motility assay, both mutant cardiac myosins exhibited a reduced ability to translocate actin (V_actin) but had similar force-generating capacities. Actin-activated ATPase activities were also reduced. Single-molecule laser trap experiments revealed that the lower V_actin in the S532P mutant was due to a reduced ability of the motor to generate a step displacement and an alteration of the kinetics of its chemomechanical cycle. These results suggest that the depressed molecular function in cardiac myosin may initiate the events that cause the heart to remodel and become pathologically dilated.