Novel mechanochemical activation generates biomass‐derived carbons with unprecedented CO2 storage capacity due to higher porosity than analogous conventionally activated carbons but similar pore size. The mechanochemical activation, or so‐called compactivation, process involves compression, at 740 MPa, of mixtures of activating agent (KOH) and biomass hydrochar into pellets/disks prior to thermal activation. Despite the increase in surface area and pore volume of between 25% and 75% compared to conventionally activated carbons, virtually all of the porosity of the biomass (sawdust and lignin) derived mechanochemically activated carbons is from small micropores (5.8–6.5 Å), which results in a dramatic increase in CO2 storage capacity at 25 °C and low pressure (≤1 bar). The ambient temperature CO2 uptake for a carbon derived from sawdust at 600 °C and a KOH/carbon ratio of 2, rises from 1.3 to 2.0 mmol g−1 at 0.15 bar, and from 4.3 to 5.8 mmol g−1 at 1 bar, which is the highest ever reported for carbonaceous materials. The mechanochemically activated carbons have a superior CO2 working capacity for pressure swing adsorption and vacuum swing adsorption processes and, due to a high packing density, they exhibit excellent volumetric CO2 uptake that is higher than for any material reported to date.