A facile mechanochemical synthesis was used for production of highly mesoporous carbon-alumina composites from sustainable main precursors such as tannins and boehmite powder. The one-pot ball milling of the precursors in the presence of a block copolymer followed by thermal treatment led to the mesoporous carbon-alumina composites with high specific surface areas (up to 558 m² g⁻¹) and large pore volumes (up to 1.47 cm³ g⁻¹). The presence of carbon component in the composites was essential to enhance their CO₂ adsorption capacities up to 1.80 mmol g⁻¹ at 1 bar and 25 °C. The presented synthesis strategy seems to have a great potential in practical uses for the preparation of mesostructured carbon-alumina composites, aluminas and carbons for applications in adsorption and catalysis. The proposed synthesis assures formation of abundant amounts of relatively large mesopores in the different classes of inorganics, although the pore order is not achieved in the presented structures.