Abstract: Developing highly stable solid amine materials for carbon capture is essential for achieving carbon emission reduction goals. Layered double hydroxides (LDHs) possess a highly dispersed spatial structure, and their calcined derivatives—mixed metal oxides (MMOs)—offer abundant slit-like mesopores and a broad pore size distribution, making them promising supports for preparing solid amine adsorbents. Mixed metal oxide Mg?.??Al-O was synthesized with a Mg/Al ratio of 0.55 and compared with the commercially available mesoporous material SBA-15. A series of solid amine adsorbents with different polyethyleneimine (PEI) loadings were prepared via the impregnation method, the CO? adsorption–desorption properties and stability were investigated under simulated flue gas conditions (15% CO?/N?). Mg?.??Al-O exhibits a high density of mesopores with pore diameters ranging from 5 to 20 nm, its pores are gradually filled with increasing PEI content. When the PEI loading exceeds 33 wt.%, the Mg?.??Al-O-based adsorbents demonstrate higher CO? adsorption capacity than the PEI-functionalized SBA-15. Both types of adsorbents reach their maximum CO? uptake at 67 wt.% PEI. In-situ FTIR reveals that Mg?.??Al-O-PEI adsorbents form carbamic acid adsorption species during CO? adsorption, enhancing the reaction efficiency between CO? and amine groups and thus improving amine efficiency. Thermogravimetric analysis at 75°C under 15% CO?/N? conditions for 60 minutes shows CO? uptakes of 3.44 and 2.81 mmol/g for Mg?.??Al-O-67PEI and SBA-15-67PEI, respectively. Unlike the hydrogen bonding between PEI and SBA-15, strong electrostatic interactions between PEI and Mg?.??Al-O contribute to superior thermal stability. High-temperature desorption experiments confirm that Mg?.??Al-O-67PEI retains its adsorption activity after treatment at 300°C. After 110 adsorption–desorption cycles, it still maintains a CO? capacity of 3.28 mmol/g, indicating excellent stability of the amine groups during long-term cyclic operation.