Current platforms for studying the mechanical properties of human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) as single cells do not measure forces directly, require numerous assumptions, and cannot study cell mechanics at different loading conditions. We present a method for directly measuring the active and passive forces generated by single-cell hPSC-CMs at different stretch levels. Utilizing this technique, single hPSC-CMs exhibited positive length-tension relationship and appropriate inotropic, klinotropic, and lusitropic changes in response to pharmacological treatments (isoproterenol and verapamil). The unique potential of the approach for drug testing and disease modeling was exemplified by doxorubicin and omecamtiv mecarbil drug studies revealing their known actions to suppress (doxorubicin) or augment (omecamtiv mecarbil at low dose) cardiomyocyte contractility, respectively. Finally, mechanistic insights were gained regarding the cellular effects of these drugs as doxorubicin treatment led to cellular mechanical alternans and high doses of omecamtiv mecarbil suppressed contractility and worsened the cellular diastolic properties.