Graphene-based nanocomposites possess excellent mechanical, electrical, thermal, optical, and chemical properties. These materials have potential applications in high-performance transistors, biomedical systems, sensors, and solar cells. This paper presents a critical review of the recent developments in graphene-based nanocomposite research, exploring synthesis methods, characterizations, mechanical properties, and thermal properties. Emphasis is placed on characterization techniques and mechanical properties with detailed examples from recent literature. The importance of characterization techniques including Raman spectroscopy, X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM) for the characterization of graphene flakes and their composites were thoroughly discussed. Finally, the effect of graphene even at very low loadings on the mechanical properties of the composite matrix was extensively reviewed.