By means of first-principles calculations based on density functional theory (DFT), DFT þ U and hybrid functional methods, we report a comparative study of the magnetic, electronic, and ferroelectric properties of high-pressureinduced compounds MnMO 3 (M ¼ Ti, Sn). The results correctly describe the insulating character and G-type antiferromagnetic ground state for both compounds, which is in good agreement with the experimental observations. We predicted large spontaneous ferroelectric polarizations of MnTiO 3 and MnSnO 3 by using the Berry-phase method. In particular, the proper covalent interaction mechanism driving the ferroelectric transition is discussed and explained in term of the analysis of potential-energy surfaces, Born effective charges, and electric localization function. Our results indicate that MnTiO 3 and MnSnO 3 represent unique examples of ferroelectric perovskites in which the ferroelectric instabilities originate from the combined action of geometric effects and chemical activity of the B-site atom, thus extending the concept of d 0 -ness (MnTiO 3 ) and lone-pair mechanism (MnSnO 3 ) to magnetic ferroelectrics.