Carbon nanomaterials were used to enhance desired properties of polymeric matrices. Combining varied carbon nanofillers with dissimilar dimensions can lead to synergy through effective dispersion. In this work, mechanical, thermal, acoustic, and dielectric properties of polyurethane (PU) hybrid nanocomposites were investigated. PU containing multi-walled carbon nanotubes (MWCNT) and graphene nanoplatelets (GNPs) were used to evaluate the effects of single and hybrid nanofillers on the final properties of nanocomposites. The results showed a synergistic effect between nanofillers, in which the hybrid nanocomposites exhibit better performance, relative to the single inclusion of the nanofillers. These hybrid nanofillers improved dispersion quality in the polymer matrix due to the formation of a GNPs/CNTs 3D architecture, in which tensile strength, thermal conductivity, acoustic transmission loss, and dielectric constant of PU were enhanced by about 43%, 5%, 51%, and 13% at 0.25 wt% loadings, respectively. The overall properties of the hybrid nanocomposite revealed the superiority over the single nanofiller system in multifunctionality, evaluated by a performance index.