In this study, nitrogen-doped microporous carbons, with potential use as capturers for CO₂, were synthesized directly from two new benzoxazines (BZCN and BZPh)—prepared from two phenols (phenol and 4-cyanophenol, respectively), CH₂O, and 1,3,5-tris(4-aminophenoxy)benzene (TPhA)—through thermal curing, carbonization, and KOH activation. FTIR and NMR spectroscopy confirmed the corresponding chemical structures; temperature-dependent FTIR spectroscopic and DSC analyses revealed the thermal curing behavior of the ring opening polymerizations of these two new benzoxazine monomers. Of the two benzoxazines, the nitrile-functionalized benzoxazine BZCN, derived from 4-cyanophenol, exhibited—after thermal curing, carbonization, and KOH activation—enhanced thermal properties, cross-linking density, and CO₂ capture, based on thermogravimetric analysis, dynamic mechanical analysis, and Brunauer–Emmett–Teller (BET) analysis. Analyses using Raman spectroscopy, wide-angle X-ray diffraction, transmission electron microscopy, the BET method, and X-ray photoelectron spectroscopy provided information about the microporous structures and surface areas of these two highly ordered microporous N-doped carbon materials.