Iron sulfide minerals are relevant in origin of life theories, due to their potential catalytic activity towards the activation and conversion of carbon dioxide (CO2) to small organic molecules, which may be applicable to the production of liquid fuels and commodity chemicals. The mechanistic details of the catalytic reactions between CO2 and H2 at iron sulfide surfaces however, remain poorly understood, although this information is critical to the development of improved iron sulfide catalysts for the conversion of CO2 into fuels and chemicals. Herein, we perform high quality periodic density functional theory (DFT) calculations of the reaction mechanisms associated with the direct and hydrogen-assisted CO2 reduction reactions on the prefect and defective surfaces of layered iron monosulfide (mackinawite). The low-index Miler surface of FeS are systematically characterized and based on the calculated surfaces energies, we have simulated the thermodynamic crystal morphology using Wulff construction. The fundamental aspects of CO2 adsorption, including the registries of the adsorption complexes, adsorption energies, electronic properties, and structural parameters are presented. It is demonstrated that the CO2 molecule physisorbs on the most stable (001) surface, but chemisorbs relatively strongly on the (011) and (111) FeS surfaces, preferentially at Fe sites. We have also examined the creation of sulfur vacancies on the FeS(001) surface and their impact on CO2 adsorption and found that the exposed Fe sites on the defective FeS(001) surface are catalytically active towards the adsorption and activation the CO2 molecule due to reduced work function. Compared to the perfect surface (001), the adsorption of the CO2 on the defective FeS(001) surface is shown to be characterized by significant charge transfer from the interacting surface Fe ions into the π-antibonding of the CO2 molecule, which induced a large structural transformation in the molecule (i.e., forming a negatively charged bent CO2−δ specie, with weaker C−O confirmed via vibrational frequency analyses). The thermodynamics and kinetics of the direct versus hydrogen-assisted CO2 reduction pathways on the defective FeS(001) and on the perfect (011) and (111) FeS surfaces were also systematically analyzed and will be discussed).