Novel metal-matrix nanocomposites (MMNCs) of Cu–30Ni–20Cr and Cu–50Ni–20Cr (by wt%), having a nanocrystalline Cu–Ni solid solution matrix with the dispersion of Cr nanoparticles, were fabricated by coelectrodeposition. Both nanocomposites exclusively grew external chromia scale during oxidation at 800 °C in air. The codeposited Cr nanoparticles, together with the numerous grain boundaries in the Cu–Ni matrix, promoted the establishment of a continuous chromia scale during the initial and transient oxidation stage, and then they functioned as “a reservoir” supplying sufficient Cr flux for the exclusive growth of the scale during the steady-state stage. The theoretical treatment using a two-phase alloy oxidation model indicates that high Cr diffusivity correlated with the persistence of an ultrafine-grained structure of the Cu–Ni matrix during oxidation is crucial to the exclusive chromia growth. Ni content increase did not significantly affect the chromia scale formation. This is fundamentally different from the oxidation of conventional Cu–Ni–Cr alloys investigated for comparison.