The last decade has seen an enormous rise in the use of green reducing agents, such as plant extracts, for the chemical synthesis of several materials in view of the limitations of conventional reducing agents, such as their toxicity and instability. This study reports the green reduction and simultaneous functionalization of graphene oxide on cellulose fibers using the aqueous extract from the inflorescences of Cannabis sativa L. The graphene oxide, synthesized using the modified Hummer’s method, was reduced in situ on the cellulose matrix in the presence of the extract at elevated temperatures without external stabilizers in order to functionalize the fibers with reduced graphene oxide (RGO). The cellulose fibers not only acted as a flexible, biodegradable, and cost-effective matrix for the anchorage of RGO but also supported in situ reduction on the fiber surface. Different weight fractions of RGO, from 0.1 to 10 wt %, were used to fabricate RGO/cellulose composites by paper-making technique, which were characterized using Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy techniques. The RGO sheets uniformly covered the surface of the cellulose fibers and dispersed well within the fiber matrix. The surface resistivity at 40 V decreased with increasing RGO content from 1.81 × 1011 Ω for 0.1 wt % RGO to 0.15 × 1011 Ω for 10 wt % RGO loading. The presence of air voids between the fibers hindered the physical contact between the RGO layers, thereby preventing the formation of an effective conductive network and significantly affecting the performance of the composites. Likewise, the surface charging capacity of the composites at 40 V dropped from 1.21 × 10−3 ΔmAh for 0.1 wt % RGO to 0.05 × 10−3 ΔmAh for 10 wt % RGO content, indicating a rise in conductivity with RGO loading. These composites show immense potential as sustainable materials for portable energy storage devices, such as capacitors.