A detailed mechanistic study of the electrochemical CO₂ reduction catalyzed by the fac-[Mn^I(CO)₃(bis-^MeNHC)MeCN]⁺ complex (1-MeCN⁺) is reported herein by combining in situ FTIR spectroelectrochemistry (SEC), synthesis and characterization of catalytic intermediates, and DFT calculations. Under low proton concentrations, 1-MeCN⁺ efficiently catalyzes CO₂ electroreduction with long catalyst durability and selectivity toward CO (ca. 100%). The [Mn^-I(CO)₃(bis-^MeNHC)]⁻ anion (1^–) and the tetracarbonyl [Mn^I(CO)₄(bis-^MeNHC)]⁺ complex (1-CO⁺) are key intermediates of the catalytic CO₂-to-CO mechanism due to their impact on the selectivity and the reaction rate, respectively. Increasing the proton concentration increases formate production (up to 15% FE), although CO remains the major product. The origin of formate is ascribed to the competitive protonation of 1^– to form a Mn(I) hydride (1-H), detected by SEC in the absence of CO₂. 1-H was also synthesized and thoroughly characterized, including by X-ray diffraction analysis. Stoichiometric reactivity studies of 1-H with CO₂ and labeled ¹³CO₂ indicate a fast formation of the corresponding neutral Mn(I) formate species (1-OCOH) at room temperature. DFT modeling confirms the intrinsic capability of 1-H to undergo hydride transfer to CO₂ due to the strong σ-donor properties of the bis-^MeNHC moiety. However, the large potential required for the HCOO^– release from 1-OCOH limits the overall catalytic CO₂-to-HCOO^– cycle. Moreover, the experimentally observed preferential selectivity for CO over formate is dictated by the shallow kinetic barrier for CO₂ binding to 1^– compared to the Mn–H bond formation. The detailed mechanistic study highlights the reduction potential, pK_a, and hydricity of the metal hydride intermediate as crucial factors affecting the CO₂RR selectivity in molecular systems.