There is a growing demand for new methods to prepare porous Si-based materials for applications in optoelectronic and microelectronic devices. In this work, the preparation of SiC and Si from magnesiothermic reduction of chiral nematic SiO2/C composites and mesoporous SiO2, respectively, is reported. The SiO2/C composites are prepared by cocondensing SiO2 with cellulose nanocrystals (CNCs) followed by pyrolysis. The magnesiothermic reduction of the composites produces SiC after prolonged solid-state reaction, with mixed MgC2/SiC intermediates. Iridescent mesoporous tetragonal MgC2/SiC structures that retain the long-range twisted organization of the starting composites transform to mesoporous cubic SiC with a chiral nematic hierarchical structure, but with some loss of order. On the other hand, the magnesiothermic reduction of the chiral nematic mesoporous SiO2 templated from CNCs affords mesoporous Si materials with a layered hierarchical structure. The structural properties and the conductivity of the products, as well as the reaction pathways by analysis of the materials at intermediate stages, are investigated. These experimental results show that the magnesiothermic reduction is a promising way to obtain new porous semiconducting materials with chiral nematic structures.