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We present calculations of the lattice constants, structural parameters, bulk moduli, energies of formation and band structures of Mg-IV-N2 compounds with IV=Si, Ge, Sn using the fullpotential linearized muffin-tin orbital method and the quasiparticle self-consistent GW approach for the wurtzite based P na21 crystal structure. The lattice parameters calculated with the generalized gradient approximation (GGA) are found to be in good agreement (within 1 %) with experiment for the cases of MgSiN2 and MgGeN2, where data are available. Similar to the Zn-IV-N2 compounds, MgSiN2 is found to have an indirect gap slightly lower than the lowest direct gap, while the other materials have direct gaps. The direct gaps, calculated at the GGA lattice constant, range from 3.43 eV for MgSnN2 to 5.14 eV for MgGeN2 and 6.28 eV for MgSiN2 in the 0.8Σ approximation, i.e. reducing the QSGW Σ by a factor 0.8 and including an estimated zero-point motion correction. The symmetry character of the valence band maximum states and their splittings and effective masses are determined. The conduction band minima are found to have slightly higher Mg-s than Si−s like character in MgSiN2 but in MgGeN2 and MgSnN2, the group-IV-s character becomes increasingly dominant.