Electrocatalysts for CO₂ reduction based on first-row transition metal ions have attracted attention as abundant and affordable candidates for energy conversion applications. Yet very few molecular iron electrocatalysts exhibit high selectivity for CO. Iron complexes supported by a redox-active 2,2′:6′,2″-terpyridine (tpy) ligand and a strong trans effect pyridyl-N-heterocyclic carbene ligand (1-methyl-benzimidazol-2-ylidene-3-(2-pyridine)) were synthesized and found to catalyze the selective electroreduction of CO₂ to CO at very low overpotentials. Mechanistic studies using electrochemical and computational methods provided insights into the nature of catalytic intermediates that guided the development of continuous CO₂ flow conditions that improved the performance, producing CO with >95% Faradaic efficiency at an overpotential of only 150 mV. The studies reveal general design principles for nonheme iron electrocatalysts, including the importance of lability and geometric isomerization, that can serve to guide future developments in the design of affordable and efficient catalysts for CO₂ electroreduction.