Abstract: By employing micrometer and ultrafine-scaled Ti₃AlC₂ MAX phase ceramics as reinforcement, porous ceramic scaffolds were manufactured via hot-press sintering of ceramic powders at varied temperatures. Subsequent pressureless infiltration of the pores of scaffolds with molten AZ91D Mg alloy resulted in the formation of interpenetrating-phase Mg-based composites reinforced by MAX phase ceramics. The phase composition, microstructure, and mechanical properties of the composites were characterized. The influence of ceramic phase content and characteristic dimensions on the mechanical properties of composites was investigated. Both the metal and ceramic phases were continuous and mutually interpenetrated with each other in the composites. The composites exhibited high hardness exceeding 1.96 GPa and flexural strength surpassing 540 MPa. An increase in ceramic content led to enhanced hardness, but with synchronous reduction in flexural strength and fracture toughness. The structural refinement of composites from micrometer to ultrafine scale yielded substantial improvements in hardness but decrease in fracture toughness. This study may offer new avenues and guidance for the exploitation of high-strength and high-hardness Mg-based composites as well as for the optimization of their structure and properties.