Ternary half-Heusler materials with 18 valence electrons show semiconducting behavior and are studiedintensively because of their promising thermoelectric properties. Quaternary half-Heusler materials withfour different atoms and containing 18 valence electrons, similar to their ternary counterparts, are alsopromising due to their low thermal conductivity. In this study, thermoelectric properties of ternary half-Heusler ScNiSb and the predicted quaternary half-Heusler MgTiNi2Sb2 materials are investigated fromfirst principles. Calculations are carried out using density functional theory (DFT) within generalizedgradient approximation (GGA). Equilibrium lattice parameters, bulk modulus, pressure dependence ofbulk modulus, electronic band structures, total and partial electronic density of states, and vibrationalproperties are calculated and results are compared with available experiments and other calculations.Thermoelectric properties such as Seebeck coefficient, electrical conductivity, and electronic thermalconductivity are calculated by considering various scattering mechanisms beyond constant relaxationtime approximation. Lattice thermal conductivities are calculated from phonon Boltzmann transportequation without using any experimental parameters. We have found that, by replacing the Sc atoms ofScNiSb with Mg and Ti, the obtained quaternary material MgTiNi 2 Sb 2 exhibits improved p-typethermoelectric performance not only because of its lower thermal conductivity but also due to itsenhanced electronic transport properties. We predicted the highest p-type ZT=1.25 value at 1000 K forMgTiNi 2 Sb 2 , which is about 2.5 times larger compared to ScNiSb. This study elucidates the promisingthermoelectric performance of quaternary half-Heusler materials.