This work reports efficient binary Fe–Mn metal oxide catalysts for low-temperature formaldehyde (HCHO) oxidation in air. The highest catalytic activity was obtained at the molar ratio of Fe/Mn = 1 : 1, which formed a unique Fe3Mn3O8 phase. The complete oxidation of HCHO on this catalyst could be reached at room temperature and maintained for 20 h at a high GHSV (150 L g−1 h−1). In situ FTIR study identified the reaction intermediates such as dioxymethylene, formate, and carbonate. The systematic characterization correlates well with the catalyst structure and the catalytic properties, e.g., the oxygen vacancies and the derived active oxygen species during the reactions, deepening the understanding of the catalysis on the binary Fe–Mn metal oxide catalysts.