Mn-promoted CeO2 is a promising catalyst for the low temperature selective catalytic reduction of NO by NH3. We investigated the mechanism of this reaction for a model in which Mn cations are doped into the CeO2(111) surface by quantum-chemical DFT+U calculations. NH3 is preferentially adsorbed on the Lewis acid Mn sites. Dissociation of one of its N–H bonds results in the key NH2 intermediate that has been experimentally observed. NO adsorption on this NH2 intermediate results in nitrosamine (NH2NO) that can then undergo further N–H cleavage reactions to form OH groups. The resulting N2O product is desorbed into the gas phase and can be re-adsorbed through its O atom on an oxygen vacancy in the ceria surface, resulting from water desorption. Water desorption is the most difficult elementary reaction step. This redox mechanism involves doped Mn as Lewis acid sites for ammonia adsorption and O vacancies in the ceria surface to decompose N2O into the desired N2 product.