Mesoporous silicon carbide (SiC) was synthesized by a one-pot thermal reduction of SiO2/C composites by metallic Mg at the remarkably low temperature of 800 °C. Two distinct mesostructured silica were used as hard templates for composite preparation: a hexagonal 3D close-packed assembly of Stöber silica spheres and an ordered mesoporous SBA15 silica. In the latter case, SiC has crystallized in its 2H-SiC hexagonal phase, which is rather unique at such a low temperature. Composites were obtained by impregnation/polymerisation/carbonisation of a molecular carbon precursor within the porous structure of the silica template. After thermal treatment at a moderate temperature in the presence of Mg and subsequent by-products rinsing off, both prepared SiC showed distinct mesoporous structures related to the initial SiO2 architectures. By comparison of the mesoporous characteristics, resulting SiC was found to retain the carbon structures of the pristine composites. The description of the synthetic mechanism of this topotactic reaction contrasts with the usual assumption stating the templating role of silica.