Carbon composite monoliths were prepared from a commercial phenolic resin mixed with just 1 wt% of carbon nanotubes (CNTs) followed by carbonization and physical activation with CO₂. The products possess a hierarchical macroporous–microporous structure and superior CO₂ adsorption properties. In particular, they show the top-ranked CO₂ capacity (52 mg CO₂ per g adsorbent at 25 °C and 114 mmHg) under low CO₂ partial pressures, which is of more relevance for flue gas applications. This matches or exceeds those of carbons produced by complex chemical activation and functionalization. Our study demonstrates an effective way to create narrow micropores through structural modification of carbon composites by CNTs.