The objective of this study was to convert biochar, a byproduct generated from the hydrothermal process (in oxygen-limited environment) of biomass (e.g., corn stover), into value-added product. In specific, three-dimensional (3D) biochar-containing precursor sponge, which was made by using electrospun polymer nanofibers as skeleton support, was fabricated via an innovative approach. The weight ratio of biochar to polymer (in the precursor sponge) was 2/1, and it appeared that the biochar weight ratio could be further increased. Upon heat treatments (i.e., stabilization in air and carbonization in argon), the precursor sponge was converted into carbon nanofibrous sponge that had the porosity of ~ 90 vol%, the BET surface area of ~ 51.7 m² g^− 1, and the carbon content of ~ 95 wt%; and it was mechanically elastic/resilient. The electrochemical study indicated that, the carbon nanofibrous sponge could be utilized for making supercapacitor electrode with excellent rate capability and high kinetic performance. This study would not only demonstrate a high-value application of hydrothermally generated biochar, but also provide a facile while novel approach for the fabrication of carbon nanofibrous sponge which could be potentially used for various applications (particularly the energy storage application).