A novel one-pot solvent-free method was developed for the first time to prepare a series of nitrogen and magnesium codoped mesoporous carbon materials (NMgCs). The current strategy involves facile mixing, grinding, and thermal treatment of structure-directing agent and precursors. The obtained functionalized carbon materials were comprehensively characterized by X-ray diffraction, a N2 adsorption isotherm, scanning electron microscopy, transmission electron microscopy, a CHN analyzer, an inductive coupled plasma spectrometer, and X-ray photoelectron spectroscopy. The results showed that high nitrogen and MgO contents were successfully incorporated into the composites, and the obtained composites exhibited highly ordered mesostructures with good textural properties, despite the fact that a slight degradation of the mesostructure was observed at high MgO loadings. The obtained composites were further employed to capture CO2 with encouraging uptakes of 2.45 mmol/g at 25 °C. Interestingly, CO2 adsorption at 75 °C still showed a capacity as high as 1.17 mmol/g. The enhanced CO2 adsorption in comparison with parent material can be attributed to the doping of highly dispersed MgO sites and nitrogen atoms in the composite that are beneficial for adsorbing the acidic CO2 molecules. Stability tests were also carried out as well which indicated negligible loss of capacity after 10 consecutive adsorption–desorption cycles. The above results convinced us that the current NMgCs may become a potential adsorbent for practical CO2 capture utilities.