A new phenoxo-bridged diMnIII complex, Na[Mn2L(OH)2(H2O)2]·5H2O (1), obtained with the ligand L5− = 5‑methyl‑2‑hydroxo‑1,3‑xylene‑α,α‑diamine‑N,N,N′,N′‑tetraacetato, has been prepared and characterized. Mass spectrometry, conductivity, UV–visible, EPR and 1H NMR spectroscopic studies showed that the complex exists in solution as a monoanionic diMnIII complex. Complex 1 catalyzes H2O2 disproportionation with second-order rate constant kcat = 305(9) M−1 min−1 and without a time-lag phase. Based on spectroscopic results, the catalase activity of complex 1 in methanol involves a MnIII2/MnII2 redox cycle, which distinguishes this catalyst from other phenoxo-bridged diMn complexes that cycle between MnIIMnIII/MnIIIMnIV species. Addition of base stabilizes the catalyst, restrains demetallation during catalysis and causes moderate enhancement of catalase activity. The terminal carboxylate donors of 1 not only contribute as internal bases to assist deprotonation of H2O2 but also favor the formation of active homovalent diMn species, just as observed for the enzyme.