Hypertrophic cardiomyopathy (HCM) is a heritable cardiovascular disorder that affects 1 in 500 people. In infants it can be particularly severe and it is the leading cause of sudden cardiac death in pediatric populations. A high percentage of HCM is attributed to mutations in β-cardiac myosin, the motor protein that powers ventricular contraction. This study reports how two mutations that cause early-onset HCM, D239N and H251N, affect the mechanical output of human β-cardiac myosin at the molecular level. We observe extremely large increases (25% - 95%) in the actin gliding velocity, single molecule intrinsic force, and ATPase activity of the two mutant myosin motors compared to wild type myosin. In contrast to previous studies of HCM-causing mutations in human β-cardiac myosin, these mutations were striking in that they caused changes in biomechanical parameters that were both greater in magnitude and more uniformly consistent with a hyper-contractile phenotype. In addition, S1-S2 binding studies revealed a significant decrease in affinity of the H251N motor for S2, suggesting that this mutation may further increase hyper-contractility by releasing active motors from a sequestered state. This report shows, for the first time, a clear and significant gain in function for all tested molecular biomechanical parameters due to HCM mutations in human β-cardiac myosin.