We report a general strategy to synthesize Fe single-atom catalysts (SACs) on various substrates (e.g., multiwalled carbon nanotubes, SiO₂, and TiO₂) with high Fe loading (>1.5 wt %) by atomic layer deposition (ALD) through optimization of ferrocene (Fe precursor) dose time. Fe/SiO₂ and Fe/TiO₂ SACs were used for CO oxidation reactions and catalytic degradation of a methylene blue (MB) solution, respectively. Compared with reported iron oxide catalysts, the efficiency of Fe/SiO₂ for CO oxidation was more than 2 orders of magnitude higher than the reported values. The photocatalysis experiment demonstrated that TiO₂ nanoparticles, deposited with two cycles of Fe ALD, showed the highest activity and had a more than 6-fold photocatalytic activity enhancement over pure TiO₂ for the degradation of MB. This universal method, optimized Fe ALD, expands the potential applications of Fe SACs on different substrates.