The manganese hydride dimer, [(2,6-iPr2PhBDI)Mn(μ-H)]2, was found to mediate nitrile dihydroboration, rendering it the first manganese catalyst for this transformation. Stoichiometric experiments revealed that benzonitrile insertion affords [(2,6-iPr2PhBDI)Mn(μ-NCHC6H5)]2 en route to N,N-diborylamine formation. Density functional theory calculations reveal the precise mechanism and demonstrate that catalysis is promoted by monomeric species.