Mg and Mg alloys, as important lightweight metal materials, have attracted great attention due to their excellent properties, such as low density, high specific strength, and good damping properties; however, their extensive applications are strictly limited by their low strength. The strength of Mg and Mg alloys can be effectively increased by introducing reinforcement phases into their matrices, i.e., via fabricating Mg-based composites. Nevertheless, the mechanical properties of Mg-based composites demonstrate a strong dependence on their microstructures. Here, new Mg-based composites reinforced by SiC whisker scaffolds with three-dimensional interpenetrating-phase architecture were fabricated through pressureless infiltration of the melt of pure Mg or AZ91D Mg alloy into the porous scaffolds of SiC whiskers. These whiskers were preferentially stacked in-plane within lamellae in the composites using gravity-assisted sedimentation and subsequent densification during the fabrication process. The microstructures and mechanical properties of the composites, particularly their fracture toughness, were characterized and analyzed. The wettability between the SiC whisker scaffolds and the melt was improved by introducing surface reactions between them which was accomplished by a pre-oxidation treatment of the scaffolds before infiltration. The pre-oxidation temperature was adjusted to ensure an adequate filling of the scaffolds without voids while avoiding the formation of excessive reaction products. The resulting composites exhibited a high flexural strength with a certain extent of fracture toughness as evidenced by stable crack propagation with rising R-curve behavior. In comparison to pure Mg, the composites infiltrated with AZ91D Mg alloy as the matrix contained a larger amount of coarsened precipitates, resulting in apparent brittleness despite increased strength.