Defects naturally abound in semiconductor crystal structures and their presence either debilitates or improves device functionality. The increasing trend to strategically implant or remove specific defects to tailor the properties in materials via defect engineering has made it imperative to not only quantify these defects in nanostructures but to do so via efficient contactless techniques. Here we report the use of an ultrafast Kerr-gated microscope system to quantify the defect density at different locations on a single nanowire. By measuring the evolution of nonlinear luminescence dynamics from a nanowire, we are able to extract the individual nonradiative recombination constants and obtain the defect density at locations along the nanowire length. This new method promises fast, reliable, and contactless characterization of single nanoparticles.