A simple approach is demonstrated to prepare functionalized spongy graphene/cobalt sulfide (FG-CoS) nanocomposites as interconnected, porous 3-dimensional (3D) network crinkly sheets. Such crinkly sheets contain the reduced spongy graphene oxide (SGO) sheets and the intercalated CoS nanoparticles within the spongy graphene. The fabricated FG-CoS composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and Raman spectroscopy. The synthesized materials were tested as supercapacitor materials in 3 M KOH using cyclic voltammetry (CV) at different potential scan rates, and galvanostatic charge/discharge tests at different current densities. The FG-CoS electrodes showed a maximum specific capacitance of 1072 F/g at a scan rate of 1 mV/s and exhibited excellent cycling retention of 117% after 1000 cycles at 100 mV/s. The obtained energy density is 35.2 Wh/kg with a power density of 250 W/kg at 1.0 A/g. This high performance can be related to the synergistic effect of graphene and CoS, where CoS is sandwiched between graphene nanosheets. This makes the FG/CoS composite promising electrode material for a high-performance supercapacitor.