Iron-nitrogen-carbon (Fe-N-C) materials featuring Fe-N4 moieties are promising candidates to replace commercial Pt/C catalysts for the oxygen reduction reaction (ORR). Here we report a novel Mn-Fe-N/S@mC catalyst comprising atomically dispersed Fe-N4 sites and a trace amount of Mn-N2S2 sites, which are homogeneously anchored on an ordered mesoporous graphitic carbon framework. This dual-metal catalyst is realized based on the chemical conversion of self-assembled Mn0.43Fe2.57O4 nanocrystal supracrystals, with the dual metal sites and the graphitic framework derived from nanocrystal cores and the associated capping ligands, respectively. When evaluated as electrocatalysts for the ORR, Mn-Fe-N/S@mC exhibits excellent catalytic activity in alkaline media, outperforming conventional Pt/C and most non-precious-metal catalysts reported previously. Notably, the synergistic effect of having Fe and Mn together improves the catalytic activity by a factor of ~ 5. To better understand the high efficiency of Mn doping in enhancing the activity of Fe-N4 sites, density functional theory calculations were carried out, revealing that the synergistic interaction between the dual metal sites reduces the *OH reduction energy barrier during the oxygen reduction pathway.