Polystyrene matrixes containing cellulose nanofibril (CNF) with fiber content of 0.5, 1, 5, and 10 wt% were successfully hydrophobized by silylation and extruded into single filaments using both single and dual heat extrusion processing. The fiber–matrix bonding was examined using a scanning electron microscope. With further characterization, Fourier transform infrared spectroscopy showed a formation of Si-O-C bonds, indicating better fiber–matrix adhesion. Raman spectroscopy showed disruption of hydrogen bonding, which indicates interference of parallel nanocellulose fiber adhesion to neighboring fibrils. Thermogravimetric analysis suggests that the thermal stability of the functionalized CNF is higher than that of the corresponding neat sample, which is resultant of stable Si bond formation. Results from dynamic mechanical analysis showed an increasing ultimate tensile strength (UTS) and elastic modulus, with peak values attributed to the dual heat processing with up 112 MPa and 10.8 GPa for the UTS and modulus, respectively. The increase is assumed to be as a result of the linear arrangement of the CNF in the Polystyrene (PS) matrix during the extrusion process. Micromechanics modeling calculations suggest the increase is moving towards the fiber properties. The results revealed the strong reinforcing ability of CNFs and their compatibility with the thermoplastic matrix if functionalized.