Mechanical stresses and strains play important roles in the normal growth and development of biological tissues, yet the cellular mechanisms of mechanotransduction have not been identified. A variety of in vitro systems for applying mechanical loads to cell populations have been developed to gain insight into these mechanisms. However, limitations in the ability to control precisely relevant aspects of the mechanical stimuli have obscured the physical relationships between mechanical loading and the biochemical signals that mediate the cellular response. We present a novel in vitro cell shearing device based on the principles of a cone and plate viscometer that utilizes microstepper motor technology to control independently the dynamic and steady components of a hydrodynamic shear-stress environment. Physical measurements of the cone velocity demonstrated faithful reproduction of user-defined input wave forms. Computational modeling of the fluid environment for the unsteady startup confirmed small inertial contributions and negligible secondary flows. Finally, we present experimental results demonstrating the onset rate dependence of functional and structural responses of endothelial cell cultures to dynamically applied shear stress. The controlled cell shearing device is a novel tool for elucidating mechanisms by which mechanical forces give rise to the biological signals that modulate cellular morphology and metabolism.