This work reports high-harmonic generation (HHG) from 2D monolayer electrides investigated by using ab-initio approach based on time-dependent density functional theory, mainly focusing on monolayer Mg2N. Due to low ionization energy and wide potential well, they exhibit distinctively high HHG efficiency and high cutoff energy for out-of-plane pumping, although they also show atomic-like behavior. For out-of-plane pumping with a peak intensity of 40 TW cm−2 at 1550 nm, high harmonics up to 120th order are generated with efficiency 4 orders-of-magnitude higher than monolayer h-BN. For pump intensity below the direct above-threshold ionization (ATI) regime, the tunneling from the ground state and ATI from one- or two-photon resonant state simultaneously contribute to HHG. In the other subnitride monolayers of alkaline earth with larger atomic sizes (Ca2N and Sr2N), further higher harmonic generation efficiencies are obtained due to the lower work functions and wider potential wells. For in-plane pumping, the intraband transition is the major source of the HHG due to the metallic in-plane responses. The presented results will attract great attention due to the peculiar HHG signature from fundamental aspects and pave the way of obtaining coherent extreme ultraviolet (EUV)/soft-X-ray with high efficiency by using monolayer electrides.