A novel type of three-dimensional ultralight aerogel sphere, consisting of one-dimensional nanocellulose-derived carbon fibers and two-dimensional graphene layers, was prepared based on a developed drop-freeze-drying followed by carbonization approach. The nanofibrous carbon efficiently prevents the agglomeration of the graphene layers, which, in turn, reduces the shrinkage and maintains the structural stability of the hybrid carbon aerogel spheres. Consequently, the aerogel spheres showing an ultralow-density of 2.8 mg/cm³ and a porosity of 99.98% accomplish the tunable dielectric property and electromagnetic wave (EMW) absorption performance. The high-efficiency utilization of biomass-derived fibrous nanocarbon, graphene, and the porous structure of the hybrid aerogel spheres leads to the excellent EMW absorption performance. The aerogel spheres display an effective absorption bandwidth of 6.16 GHz and a minimum reflection loss of −70.44 dB even at a filler loading of merely 3 wt.%, significantly outperforming that of other biomass-derived carbon-based EMW absorbing materials. This work offers a feasible, facile, and scalable approach for fabricating high-performance and sustainable biomass-based aerogels, suggesting a tremendous application potential in EMW absorption and aerospace.