One of the key problems to overcome, in the development of electronic substrates, is that of thermal distortion because of thermal mismatch between silicon and the substrate. The aim of this work is to design a mullite-cordierite composite material, with coefficient of thermal expansion tailored to silicon substrate applications. Dense, hard, and thermally stable Al₆Si₂O₁₃–Mg₂Al₄Si₅O₁₈ composite was produced by sintering amorphous precursor powder synthesized through the sol–gel method. Si(C₂H₅O)₄, Al(NO₃)₃.9H₂O, and Mg(NO₃)₂.6H₂O were used, as source of SiO₂, Al₂O₃, and MgO oxides, respectively, to prepare mullite-cordierite precursor powders. Fourier-Transform Infrared spectroscopy (FTIR), Thermogravimetry (TG), Dilatometry, Differential Thermal Analysis (DTA), and X-ray powder Diffraction (XRD) methods were used to characterize the synthesized amorphous powder and its crystallization. The microstructure of specimens sintered at 1600 °C for 1 h was analyzed using a scanning electron microscope (SEM). The hardness (HV) and coefficient of thermal expansion (CTE) of the composite sintered at 1600 °C were measured by using a hardness tester and a dilatometer, respectively. The results show the increase in density and decrease in open porosity with the increase in temperature and equivalent amount of cordierite. For specimens sintered at 1600 °C, the increase in cordierite content from 0 to 40 wt.% increased the HV from 9.18 to 13.08 GPa; a further increase to 50 wt.% decreased it to 11.15 GPa. Sample containing 40 wt.% cordierite had the highest value of hardness (HV = 13.08 GPa), representing an increase of 42.48% with respect to monolithic mullite. The CTE of the composites (in the range 50–1000 °C) showed continuous decrease from 5.23 × 10^–6 to 2.26 × 10^–6 K⁻¹ with the increase in cordierite content from 0 to 50%. Sample containing 50 wt.% cordierite displayed the lowest thermal expansion (CTE of 2.26 × 10⁻⁶ K⁻¹), representing a decrease of 56.78% with respect to monolithic mullite.