The incorporation of graphene nanosheets (GNSs) into a polymer matrix can effectively enhance its thermal and mechanical properties. We report a facile and eco-friendly method for preparing polymer nanocomposites with homogeneously dispersed GNSs in the poly(methyl methacrylate) (PMMA) matrix via first grafting functionalized graphene oxide (GO) using PMMA miniemulsion, then melt blending the grafted GO with PMMA matrix, and simultaneous in situ thermal reduction of GO in the matrix. The results show that the GNSs exhibit exfoliated morphology and good distribution in the obtained nanocomposites. A 37.9% enhancement in tensile strength and a 61.4% increase of Youngs modulus with respect to the polymer matrix are achieved by incorporating only 1.5 wt % GNSs loading. The experimental derived Youngs modulus agrees well with the theoretical simulation. Moreover, the storage modulus of the nanocomposites increases by 45%, while the glass transition temperature (Tg) increases by 7.5 degrees C at 1.5 wt % GNSs loading.