In this paper, a practical and cost-effective processing route, in situ reactive infiltration technique, was utilized to fabricate magnesium matrix composites reinforced with a network of TiC–TiB2 particulates. These ceramic reinforcement phases were synthesized in situ from Ti and B4C powders without any addition of a third metal powder such as Al. The molten Mg alloy infiltrates the preform of (Tip + B4Cp) by capillary forces. The microstructure of the composites was investigated using scanning electron microscope (SEM)/energy dispersive X-ray spectroscopy (EDS). The compression behavior of the composites processed at different conditions was investigated. Also, the flexural strength behavior was assessed through the four-point-bending test at room temperature. Microstructural characterization of the (TiB2–TiC)/AZ91D composite processed at 900 °C for 1.5 h shows a relatively uniform distribution of TiB2 and TiC particulates in the matrix material resulting in the highest compressive strength and Young's modulus. Compared with those of the unreinforced AZ91D Mg alloy, the elastic modulus, flexural and compressive strengths of the composite are greatly improved. In contrast, the ductility is lower than that of the unreinforced AZ91D Mg alloy. However, this lower ductility was improved by the addition of MgH2 powder in the preform. Secondary scanning electron microscopy was used to investigate the fracture surfaces after the flexural strength test. The composites show signs of mixed fracture; cleavage regions and some dimpling. In addition, microcracks observed in the matrix show that the failure might have initiated in the matrix rather than from the reinforcing particulates.