Polyethylene graphitic nanoplatelets (PEGN), synthesized through a mechanochemical reaction between graphite and a compatible polymer such as polyethylene, act as effective fillers for high-density polyethylene (HDPE). Analytical results indicate that graphite can be successfully exfoliated into PEGN, imparting excellent properties to the resulting material. Accordingly, PEGN & HDPE-5 nanocomposites produced through a solution process exhibit significantly improved mechanical properties, with the tensile strength and Young's modulus increasing by 30.8% and 17.3%, respectively, compared to pure HDPE. These improvements are attributed to the excellent compatibility of PEGN functionalized with ethylene groups with the HDPE matrix, leading to strong interfacial bonding and efficient stress transfer. Therefore, PEGN is shown to be a promising new reinforcing additive for a range of polymer systems. Furthermore, this approach demonstrates that using a polymer of the same type as the target matrix as a reactant can yield fillers with improved compatibility. Notably, employing solid-state polymers in this process offers a safer and more manageable alternative to liquid or gaseous reactants.