Eglin steel, an ultra-high-strength steel developed for various demanding applications, is a low-cost alternative to 4340 steel, in which nickel and cobalt additions are replaced by higher tungsten additions, thereby achieving comparable strength and ductility. Friction stir processing (FSP) was carried out on this steel under two heat input conditions, which fell above the A3 transformation line. Microhardness values along the horizontal and vertical directions of the processed region cross section were reported for the lower heat input condition, and correlated with corresponding microstructures. A 3D heat transfer and material flow model was used to predict the peak temperature and cooling rates in these zones. Site-specific tensile tests of specimens extracted from the top to the bottom of the stir zone (SZ) showed ultimate tensile strength (UTS) greater than 2 GPa, with a total elongation close to 10% at ~ 4 mm from the top surface of SZ. Transmission electron microscopy (TEM) analysis of the high strength location showed microstructure consisting of nano-twinned martensite and nano-bainite laths of size ranging from 200 nm to 300 nm, and confirmed the existence of retained austenite. This mixed microstructure was comprised of finer aggregates of martensite, bainite and retained austenite, which were postulated to be responsible for the high strength and ductility combinations.