Metal oxide catalysts with different cerium and manganese ratios were synthesized by a co-precipitation method to remove HCN and NO from exhaust gas. Ce1Mn2 exhibited the highest HCN conversion and NO removal effect with a maximum N-2 selectivity of 72%. The existence of NO can promote the conversion of HCN at low temperatures, which may be due to the reaction between the two substances forming formyl and nitrogen. CO was not detected as a product over the entire reaction temperature range. However, high concentrations of CO2, NH3, and a small amount of NQ were detected. Results indicate that HCN was first oxidized to isocyanate followed by isocyanate further reacting with H2O to generate NH3 and CO2. Part of NO was removed by the selective catalytic reduction (SCR) and oxidation reaction. Catalyst characterization showed that Ce1Mn2 exhibited a higher specific surface area, with manganese oxide mainly existing in an amorphous state, potentially forming a solid solution structure with cerium oxide. The introduction of a suitable amount of cerium was beneficial, increasing the concentration of chemisorbed oxygen on the catalyst surface. The catalyst also exhibited the properties of adsorbing H2O, which was helpful for the isocyanic acid hydrolysis reaction.