The paper addresses the difference in electrical conductivities and rheological properties between two nanocomposites of reduced graphene oxide nanosheets (rGON) with commercial ultrahigh molecular weight polyethylene (C-PE) and a low-entanglement-density UHMWPE synthesized under controlled conditions (Dis-PE). It has been found that composites made with Dis-PE can reach conductivities at least 100 times higher than those made with C-PE on doing thermal treatment at lower temperatures. However, the difference in the electrical conductivity diminishes when both sets of samples are given a high temperature treatment. This phenomenon is attributed to the difference in morphology of the polymer matrices, for example, grain boundaries between the nascent particles. Furthermore, rheological analyses of the two sets of UHMWPE/rGON nanocomposites conclusively demonstrate differences in the interaction between polyethylene chain segments of the disentangled UHMWPE and rGON, compared to the entangled commercial UHMWPE. Both composites show minima in the storage modulus at a specific graphene composition. The strong interaction of polyethylene chains with the filler inhibits disentangled UHMWPE to achieve the thermodynamic equilibrium melt state, whereas in the commercial sample, having a broader molar mass distribution, the higher adhesion probability of the long chains to the graphene surface lowers the elastic modulus of the polymer melt. Correlation between the percolation threshold for electrical conductivity and rheological response of the composites has also been discussed.