A structural modification strategy to improve the photocatalytic performance of a series of cerium(III) bis(guanidinate) mono(amide) molecular luminophores was demonstrated. Reducing the steric bulkiness of the amide ligand gave rise to two categories of complexes with distinct photophysical and photochemical properties. A structural parameter, the amide cone angle (θ), was applied to differentiate the two categories. Complexes with smaller cone angles (θ < 156°, category B) exhibited more reducing potentials and faster electron-transfer rates than those of complexes with larger cone angles (θ > 173°, category A). And only complexes in the category B could achieve the photocatalytic phenylation of an aryl bromide. These results demonstrated that reducing the steric bulkiness of the amide ligand improved the performance of cerium(III) bis(guanidinate) mono(amide) photocatalysts in a systematic manner.