Some variations provide a metal matrix nanocomposite composition comprising metal-containing microparticles and nanoparticles, wherein the nanoparticles are chemically and/or physically disposed on surfaces of the microparticles, and wherein the nanoparticles are consolidated in a three-dimensional architecture throughout the composition. The composition may serve as an ingot for producing a metal matrix nanocomposite. Other variations provide a functionally graded metal matrix nanocomposite comprising a metal-matrix phase and a reinforcement phase containing nanoparticles, wherein the nanocomposite contains a gradient in concentration of the nanoparticles. This nanocomposite may be or be converted into a master alloy. Other variations provide methods of making a metal matrix nanocomposite, methods of making a functionally graded metal matrix nanocomposite, and methods of making a master alloy metal matrix nanocomposite. The metal matrix nanocomposite may have a cast microstructure. The methods disclosed enable various loadings of nanoparticles in metal matrix nanocomposites with a wide variety of compositions.