We used transient biochemical and structural kinetics to elucidate the molecular mechanism of mavacamten, an allosteric cardiac myosin inhibitor and a prospective treatment for hypertrophic cardiomyopathy. We find that mavacamten stabilizes an autoinhibited state of two-headed cardiac myosin not found in the single-headed S1 myosin motor fragment. We determined this by measuring cardiac myosin actin-activated and actin-independent ATPase and single-ATP turnover kinetics. A two-headed myosin fragment exhibits distinct autoinhibited ATP turnover kinetics compared with a single-headed fragment. Mavacamten enhanced this autoinhibition. It also enhanced autoinhibition of ADP release. Furthermore, actin changes the structure of the autoinhibited state by forcing myosin lever-arm rotation. Mavacamten slows this rotation in two-headed myosin but does not prevent it. We conclude that cardiac myosin is regulated in solution by an interaction between its two heads and propose that mavacamten stabilizes this state.