The performance of MoS₂ based devices is closely related to the quality and defect morphology of the monolayer MoS₂ deposited on metal. First-principles calculations were performed to investigate the vacancy effects of Au–mMoS₂ contact. Four possible S-vacancy and a Mo-vacancy were considered in our calculations. Energetic studies show that S-vacancies are easier to form than Mo-vacancy in Au–mMoS₂ contact, while S-vacancy (hollow site at interface, V_S4) has the lowest formation energy under Mo-rich environments. Electron and charge redistribution analysis of defective Au–mMoS₂ contact indicate that the lower contact resistance and higher electron injection efficiency of defective Au–MoS₂ contact than perfect ones. Notably, the S-vacancy at top layer showed better electronic performance than that at bottom layer of monolayer MoS₂ in the contact. High quality n-type Au–mMoS₂ contact can therefore be expected through defect engineering.