The key to develop efficient catalysts is to improve the quantity and activity of catalytic sites of the catalysts through optimal structural and compositional design. Accordingly, open-mouth N-doped carbon nanoboxes embedded with mixed metal phosphide nanoparticles are fabricated from monodisperse Ni₃[Fe(CN)₆]₂·H₂O nanocubes through conformal Ni₃[Co(CN)₆]₂·12H₂O layer coating, ammonia etching, and thermal phosphorization, sequentially. The product catalyst exhibits highly efficient electrocatalytic performances, achieving low overpotentials of 204 and 129 mV for the oxygen evolution reaction and hydrogen evolution reaction, respectively, and a small working voltage of 1.57 V for the overall water splitting, all at 10 mA cm⁻². Its long-term electrocatalytic stability is also outstanding, experiencing only minor chronopotentiometric decay after a 24 h operation at 10 mA cm⁻². The enhanced electrocatalytic performance may be attributed to the synergistic effects between the mixed metal phosphides, the protective function offered by the chainmail catalyst design, and the fast mass transport channels for the electrolyte and gaseous products afforded by the large openings on the nanobox shell, as well as the easy access of the inner active sites of the nanobox. The ingenious open-mouth nanobox structure together with embedded mixed metal phosphide nanoparticles is a unique design for improving the quantity and activity of catalytic sites of the catalyst for high efficiency electrolytic water splitting. The present design concept can be readily applied to the fabrication of other heterogeneous catalysts.