The unprecedented sensitivity and partial selectivity of quinoxaline-walled thioether-legged deep cavitand functionalized multiwall carbon nanotubes toward traces of benzene vapors are presented. The cavitand is grafted onto gold nanoparticle (Au-NP) decorated oxygen plasma treated multiwall carbon nanotubes (O-MWCNT) by a self-assembled monolayer process affording a product referred to as cav-Au-MWCNT. The reported technique is suitable for the mass production of hybrid nanomaterials at low cost. The cav-Au-MWCNT resistive gas sensor operates at room temperature and shows an outstanding performance toward traces of benzene vapors. The detection of 2.5 ppb of benzene in dry air is demonstrated with a limit of detection (LOD) near 600 ppt. For the first time, it is shown that a CNT nanomaterial can effectively sense the extremely harmful benzene molecule with higher sensitivity than toluene or o-xylene at the trace levels. The cavitand is well suited for binding benzene, which, being in close proximity to the MWCNT, affects its density of states (DOS) shifting the Fermi level away from the valence band. The binding of benzene is transduced in a diminution of MWCNT conductance. Furthermore, the inclusion of benzene is fully reversible at room temperature, implying that the sensor can operate at very low power consumption.