Adding fillers reinforces the modulus of the polymer, but the ductility is sacrificed. Such a phenomenon is especially true for brittle polymers like polystyrene (PS). Nanofillers are known to solve the problem because the high aspect ratio and surface area may improve the stiffness at a much lower loading level, and thus significantly reduce the ductility. However, the dispersion of nanofillers is always challenging. In the current investigation, PS nanocomposites were prepared by melt compounding, ultrasonication, and solution compounding to investigate the effect of the addition of the following carbon nanomaterial to the mechanical properties: nanographite (H25), multiwalled carbon nanotube, carbon nanofiber, and thermally reduced graphene (TRG). Among them, TRG exhibited the best reinforcing effect. Young's modulus of PS increased by 27% with only 0.1 wt% loading of TRG, and the ductility remained unchanged. Such an improvement has rarely been reported in thermoplastic polymer nanocomposites. The results were carefully analyzed and compared with those reported in the available literature. In this study, the solution compounding method offered the best modulus value. The aspect ratio of carbon nanomaterials in the composite was estimated by the Halpin-Tsai equation. These estimations agree well with the transmission electron microscopy microstructure results.