Mid-temperature thermoelectric applications include waste heat recovery from automobile exhausts, various industrial process and solar thermoelectrics. The current systems that have been extensively investigated for mid-temperature applications are mostly tellurides. However, the low crustal abundance of tellurium contributes significantly to their price volatility which impedes their usage in commercial technology. Looking from the perspective of material cost effectiveness in terms of crustal abundance and environmental sustainability, we investigate the thermoelectric properties of Mg₉Si₅ systems, the constituents of which are geo-abundant and non-toxic. By means of first principles calculations, we find Mg₉Si₅ to be a potential mid-temperature thermoelectric material, with an operational temperature in the range of ≃400–600 K, depending on the exact carrier concentration. We also discuss the enhanced viability of substitution in Mg₉Si₅, a case that seems to be restricted in Mg₂Si because of its chemically balanced Zintl nature.