Metal matrix nanocomposites (MMNCs) synthesized by the inexpensive and scalable method of stir mixing have received relatively little attention due to the perceived difficulty of dispersing nanoparticles (NPs) in molten metal. However, matrix/reinforcement reactions may be useful in deagglomeration of the particles. A review of previous experimental studies shows that solid solution, Orowan, and grain boundary (GB) strengthening are most likely to influence the strength of reactive stir-mixed Al-Mg-Al2O3np MMNCs. Matrix/reinforcement reactions, porosity in stir-mixed MMNCs, and NP incorporation on grain size are also discussed. Analysis of variation of grain size with NP concentration shows that the MMNCs of this study do not follow the trend of MMNCs strengthened primarily by GB strengthening, and transmission electron microscope shows that individual NPs and agglomerates were present within the aluminum alloy matrix. This evidence coupled with strength beyond what would be expected from solid solution and GB strengthening indicate that Orowan strengthening is likely present-though significant gas porosity in low Mg concentration MMNCs is often detrimental.