Nanocarbon materials, such as carbon nanotubes, graphene, and their derivatives, are regarded as promising reinforcing agents in metal matrix composites (MMCs) because of their excellent intrinsic mechanical properties. Considering the various types of nanocarbons with different defect states and intrinsic properties, there is a potential for tailoring the mechanical behavior of nanocarbon-reinforced MMCs. This article reviews recent developments in both the processing and the structure-property correlations of these composites. Particular emphasis is given to the structure and properties of the nanocarbon-metal interfaces, as the external mechanical load is transferred between the nanocarbon and the metal matrix across their interfaces. Moreover, in addition to the intuitive load-bearing effect of the nanocarbon reinforcements, a copious interplay between nanocarbons and dislocations in the metal matrix has been found, which alters the deformation behavior that leads to additional strengthening. For structural applications, scalable fabrication routes for the nanocarbon-metal composites need to be developed, and studies on the mechanical behavior under real service conditions are needed.