Carbon nanocapsules doped separately with epoxy and polyamine were used to fabricate an epoxy nanocomposite coating. Carbon nanospheres with dual-encapsulated epoxy/polyamine were dispersed uniformly in the epoxy resin at concentrations of 2, 5, and 10 wt %. The mechanical properties of the nanocomposites were studied by tensile testing and scratch hardness measurements. Furthermore, nanocomposites were applied on mild steel substrates, and their corrosion protection and barrier performance were evaluated using electrochemical impedance spectroscopy (EIS). Adhesion loss measurements of coatings after 240 h exposure to 3.5 wt % NaCl solution were performed by pull-off adhesion testing. Also, the buried steel/polymer interface was studied in situ using attenuated total reflection Fourier transform infrared spectroscopy in the Kretschmann geometry. The results showed that both mechanical and corrosion protection properties of the prepared nanocomposites are enhanced as compared to the baseline epoxy coating and improved with the concentration of doped carbon nanocapsules. Maximum mechanical and corrosion protection properties were achieved in the case of 10 wt % doped carbon nanocapsules as a result of active intermolecular interactions between the epoxy and polyamine chains of the coating matrix and amine groups grafted on the surface of carbon capsules.