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The uniform distribution of carbonaceous nanomaterials in a magnesium matrix composite can dramatically improve the strength without sacrificing ductility. In this work, graphene oxide (GO) reinforced AZ31 magnesium alloy matrix composites were fabricated using friction stir processing to achieve higher strength, toughness, and plasticity compared with the unreinforced condition. The composite microstructure consisted of individual GO nanoplatelets and GO micro-clusters, having different effects on the deformation behavior of the matrix. Texture analysis revealed that adding GO led to the activation of different twin types and variants during tensile deformation. In contrast, only one extension twin variant was dominant in the case of the unreinforced condition. In spite of the GO micro-clusters, the individual GO nanoplatelets cannot hinder the growth of twins. The GO micro-clusters inhibit twin growth, change the stress state in the parent grains, and provide a driving force for twin nucleation. The extensive twinning occurrence was followed by twin intersections and significantly enhanced the plasticity of the alloy by providing easy orientations for further basal slip.