Abstract: High-performance composites were made with sheet graphene (GNP) and spherical silica (SiO2) as mesoporous nanofillers. The nanofillers are composed of SiO2 tightly wrapped on GNP by the sol–gel method, resulting in SiO2@GNP (SG) composites. The SG composites are then combined with epoxy resin (EP) to form polymer matrices. After conducting a series of characterization and assessments, including SEM, XRD, FTIR, and BET, the results show that the use of SG composites as hollow mesoporous nanofillers have a higher potential to create a significant quantity of thermally conductive pathways in the polymer matrix. Meanwhile, SiO2@GNP/EP (SGE) composites have low dielectric constants and dielectric losses, along with high thermal conductivities. The optimal dielectric constant of SGE composites is 2.65 at 10 Hz, achieved at a 5 wt% SG composites loading, and the dielectric losses stay below 0.02 (10–106 Hz). Furthermore, the thermal conductivity reaches 0.72 w/(m K). Compared to neat EP, the SGE composites reduce the dielectric constant by 13.7% (10 Hz) and increase thermal conductivity by 140%. The robust electrical insulation and thermal conductivity of the aforementioned features lend themselves well to a range of applications. The SGE composites show good promise in several industries, with excellent potential in the realm of 5G communications particularly, such as epoxy plastic sealing materials.