We present a method for measuring the mechanical response of a single cell in-situ when local deformation is applied at an adhesion site(s) by a functionalized MEMS (Micro Electro Mechanical Systems) force sensor with pN - nN force resolution, and with force and displacement ranges of 100s of nNs and μms. The force sensor is a micro mechanical cantilever beam made of single crystal silicon (SCS), coated by a thin layer of Fibronectin, an extra cellular matrix (ECM) protein, to activate cell adhesion. The end of the beam is brought in contact with a cell to form the adhesion site(s). The cantilever is then moved away from the cell to locally deform it. The force on the cell is measured from the deformation of the cantilever until the adhesion sites fails. We demonstrate the method by deforming several endothelial and fibroblast cells. Force response of the fibroblast cell shows linear behavior.