Catalytic and selective oxidation of monosaccharides into value-added sugar acids is highly desirable from both scientific and industrial perspectives. Here, we report a hierarchical porous carbon (NC-3) supported gold catalyst (Au/NC-3) and its application in the base-free aerobic oxidation of D-glucose and D-xylose into value-added sugar acids under mild conditions (100 °C). The catalyst exhibited excellent catalytic performance with the yields of D-gluconic acid and D-xylonic acid reaching 97.62% and 98.76%, respectively. We studied the effect of the ball-milling process on the hierarchical porous preparation, which is critical to the formation of the fibrous structure during the pyrosis process. This specific structure provides active sites for Au adsorption. DFT calculation revealed that the Au NPs prefer to anchor on pyridinic N sites; meanwhile, the initial reactants and essential intermediates are found to be well stabilized by Au/NC-3. Moreover, upon recycling, gold was stable toward the recovery procedure. In this work, we provide a green method for an improved generally selective oxidation of D-glucose and D-xylose using Au/NC-3 as an efficient heterogeneous catalyst.