In this paper, nitrogen (N)-doped ultra-porous carbon derived from lignin is synthesized through hydrothermal carbonization, KOH activation, and post-doping process for CO2 adsorption. The specific surface areas of obtained N-doped porous carbons range from 247 to 3064 m2/g due to a successful KOH activation. N-containing groups of 0.62–1.17 wt% including pyridinic N, pyridone N, pyridine-N-oxide are found on the surface of porous carbon. N-doped porous carbon achieves the maximum CO2 adsorption capacity of 13.6 mmol/g at 25 °C up to 10 atm and high stability over 10 adsorption/desorption cycles. As confirmed by enthalpy calculation with the Clausius–Clapeyron equation, an adsorption heat of N-doped porous carbon is higher than non-doped porous carbon, indicating a role of N functionalities for enhanced CO2 adsorption capability. The overall results suggest that this carbon has high CO2 capture capacity and can be easily regenerated and reused without any clear loss of CO2 adsorption capacity.