Polystyrene matrix nanocomposites were formulated using a custom nano particle consisting of nanodiamond covalently bonded to graphene oxide. Dispersion and mechanical property results for the nano composite are compared to those results for the neat polymer as well as for a nanocomposite infused with graphene oxide only. Dynamic light scattering was performed to determine the size of particles and the results showed that the custom nanoparticle reduced agglomeration by about 50% as compared to the graphene oxide alone. Microscale Vickers hardness testing revealed that neat polymer as well as the two nanocomposite samples all have similar hardness while nanoscale atomic force microscopy revealed that the neat polymer samples have the highest stiffness on average and the custom nanoparticle composite samples have the lowest stiffness. This difference in mechanical behavior with scale is attributed to local defects at the particle/matrix interface.