The demand for polymer nanocomposites is mushrooming in the global market due to their superior mechanical properties, such as specific tensile, impact, and flexural strength. Incorporating nanofillers in various polymer matrices have proved their potential to impart synergistic values to the aforementioned mechanical properties. This book chapter focuses discussion of various nanofillers, their associated polymer nanocomposites, their mechanical properties, and their specific applications. In particular, nanofillers such as nano-clays, nano-oxides, carbon nanotubes, and other nanoparticles have been discussed. Owing to the nanometric scale of fillers, using even a minimal quantity may significantly improve the composite properties such as hardness, tensile modulus, strength, and toughness at both low and high temperatures. The enhancement of such mechanical properties can be achieved due to improvement in the composite’s crystallinity without affecting the polymer’s processability by incorporating nanofillers to develop novel polymer nanocomposites. In addition, the aspect ratio of nanofillers and interfacial strength between polymer and nanofiller play a vital role in deciding the desired mechanical properties and specific applications. This book chapter will give a clear understanding to industrialists and researchers working in concerned fields to tailor the mechanical properties of polymer nanocomposites using various nanofillers.