Adsorption ability and reaction rate are two essential parameters that define the efficiency of a catalyst. Herein, we study the effects of the external electric field F on the catalytic decomposition of N2O on a Mn-embedded graphene system (Mn/graphene) based on density functional theory calculations. Our study demonstrates that Mn/graphene has a better adsorption ability than the corresponding typical catalysts, such as platinum group metals, while the appropriate positive F can make the N2O decomposition spontaneously occur via a two-step mechanism of N2O → N2 + O and N2O + O → N2 + O2. In addition, F simultaneously facilitates O2 desorption and regeneration of the Mn/graphene system, completing the whole catalytic cycle. The high synergetic catalytic effect may be attributed to that F induces an enhancement of the charge transfer between N2O and Mn/graphene. Thus, the Mn/graphene system together with the synergy of F is a good candidate for N2O adsorptive decomposition.