Manganese tricarbonyl diimine complexes bearing pyridine and imidazole ligands have been prepared as electrocatalysts for proton reduction using acetic acid as the proton source. The electron-donor ability of the diimine ligand is found to play an important role in determining the efficiency of the electrocatalysts with [MnBr(pybz)(CO)3] (pybz = 2-(2-pyridyl)benzimidazole) exhibiting the lowest overpotential (0.28 V) toward proton reduction. The [Mn(pybz)(CO)3(MeCN)]+ cationic complex prepared via debromination of [MnBr(pybz)(CO)3] by a silver salt has also been shown to catalyze proton reduction upon its electrochemical reduction. A neutral complex [Mn(pyridine-benzimidazolate)(CO)3(MeCN)], which can be synthesized by reacting [MnBr(pybz)(CO)3] with a strong base, has been detected using IR-SEC (infrared spectroelectrochemistry) as an intermediate species in the catalytic process. Using [MnBr(pybz)(CO)3] as the model electrocatalyst, we have carried out density functional calculations to propose a proton reduction mechanism consistent with our experimental observations.