The present paper is concerned with the fabrication of fully dense B4C-Mg, B4C-AZ91 alloy, and B4C-Mg,Si (eutectic) composites at a significantly lower (750–1000 °C) temperature range. The key feature of the novel approach is based on the Mg-vapor atmosphere, under which the infiltration process is carried out. This approach has also been used to fabricate B4C-Al composites at a low temperature. The microstructure and the mechanical properties of the composites are described and discussed. It was concluded that the formation of the deleterious aluminum carbide (Al4C3) depends on the presence of free carbon in the boron carbide powders. The suggested approach for low temperature fabrication of reaction bonded composites under Mg vapor can be further expanded to other boride and carbide phases.