Three new cobalt complexes of the general formula [Co(II)(L)(CH3CN) ]2+, where L is one of three pentadentate nitrogen-donor ligands based on the N4Py framework, have been synthesized and characterized. The capacity of the three complexes to effect photocatalytic proton reduction has been examined. Their photocatalytic activities in the presence of [Ru(bpy)3]2+, acting as a photosensitizer, and ascorbic acid, acting as a sacrificial electron donor, were screened in a water/acetonitrile mixture. The photochemical mechanism, as revealed by nanosecond time-resolved transient absorption spectroscopy, involves reaction of the excited sensitizer with ascorbic acid to yield [Ru(bpy)3]+ as a primary photogenerated reductant, capable of electron transfer to the cobalt catalyst(s). Under the experimental conditions used, partial decomposition of both the sensitizer and the catalyst is the main deactivation channel for photocatalysis. Optimization of reaction conditions indicated that the use of more reducing iridium or copper-based photosensitizers had a beneficial effect on the catalytic performance. The effect of the different ligands on the catalytic activities of the corresponding cobalt complexes have been investigated by DFT calculations.