This work studies the reaction mechanism and properties of reactive blends possessing a maleimide-containing benzoxazine compound (MI-Bz) and an amine-capped aniline trimer (ACAT). For the blends of MI-Bz and ACAT undergoing thermally induced reactions, the quinoid reaction of ACAT occurs first at about 96 °C, and the Michael addition reaction between the amine groups of ACAT and the maleimide group of MI-Bz follows at higher temperatures. The amine group of ACAT catalyzes the ring-opening reaction of benzoxazine groups of MI-Bz. The active species in the ring-opening reaction of benzoxazine groups are reactive toward the amine groups of ACAT. These two reactions build up the covalent bonding between ACAT and MI-Bz compounds. The thermally cured MI-Bz/ACAT mixtures (CR-MI-Bz/ACAT) demonstrate good thermal stability, high flame retardancy, and a synergistic effect on their respective anticorrosion properties. The water resistance (from MI-Bz) and the electroactivity (from ACAT) of the CR-MI-Bz/ACAT materials contribute to their high anticorrosion efficiencies. The sample possessing 33 wt% ACAT shows a protection efficiency of 98% and a corrosion rate of 4.8 μm per year for a cold-rolled steel substrate, which is about one order of magnitude lower than that recorded with the neat ACAT sample.