Amine sorbent technology offers transformative low-energy potential for both direct air capture (DAC) and post-combustion carbon capture (PCC); its viability is severely undermined by the poor heat and mass transfer of conventional solid sorbents. Herein, we report a rationally designed core-shell microwave-responsive composite support that synergistically integrates a highly graphitized carbon nanotube (CNT) core and a mesoporous silica (MS) shell for ultra-low-energy carbon capture. The CNT core not only provides efficient microwave-to-thermal conversion but also constructs a loose fibrous skeleton that prevents silica agglomeration and creates hierarchical mass transfer pathways. The MS shell, synthesized via a dual-template method, enables uniform dispersion of polyethyleneimine (PEI) even at 70 wt% loading and enhances heat transfer from the CNT core to active amine sites. The optimized PEI70-MS/CNT06 adsorbent delivers exceptional CO2 adsorption capacities of 3.28 mmol·g−1 under PCC and 2.64 mmol·g−1 under DAC conditions. Importantly, it realizes ultra-fast regeneration under 20 W microwave irradiation for only 15 s, with regeneration energy consumptions of 1.64 MJ·kg−1 CO2 (PCC) and 3.28 MJ·kg−1 CO2 (DAC), which are 60–70% lower than conventional thermal swing adsorption (TSA). The adsorbent retains 76% of its initial capacity after 50 consecutive cycles, outperforming both PEI/CNT and TSA-regenerated counterparts. This work explores microwave-assisted regeneration for high-performance DAC systems and further establishes a universal design principle for the fabrication of microwave-responsive solid amine adsorbents toward efficient dilute CO2 capture.