The strain rate mechanical behavior of 12-micron long polymeric nanofibers was investigated. Experiments were carried out by a novel method that employs a MEMS-based leaf spring load cell attached to a polymeric nanofiber that is drawn with an external PZT actuator. The elongation of the fiber and the deflection of the load cell were calculated from optical microscopy images by using Digital Image Correlation (DIC) and with 65 nm resolution in fiber extension. The nanofibers were fabricated from electrospun polyacrylonitrile (PAN) with MW = 150,000 and diameters between 300-600 nm. At strain rates between 0.00025 s−1 to 0.025 s−1 the fiber ductility scaled directly with the rate of loading while the tensile strength was found to vary non-monotonically: At 0.00025 s−1 material relaxations allowed for near-uniform fiber drawing with up to 120% ductility and 120 MPa maximum tensile strength. At the two faster rates the tensile strength scaled with the rate of loading but the fiber ductility was the result of a cascade of localized deformations at nanoscale necks with relatively constant wavelength for all fiber diameters.