A new mononuclear Ni(II) complex, [NiL] (1) of an unsymmetrically dicondensed N₂O₂ donor ligand, H₂L (where H₂L = N-α-methylsalicylidene-N′-salicylidene-1,3-propanediamine), has been synthesized. Complex 1, on reaction with Mn(II) in the presence of different co-anions, yielded three heterometallic Ni^II–Mn^II complexes, [(NiL)₂Mn(NCS)₂]·CH₃CN (2), [(NiL)₂Mn(N₃)(H₂O)](ClO₄)·H₂O (3) and [{(NiL)₂Mn(H₂O)}₂(μ_1,3-N₃)](ClO₄)₃ (4). Single crystal structural analyses show that complexes 2 and 3 contain similar bent trinuclear Ni^II₂Mn^II structures, with the difference that in complex 2 two SCN⁻ ions and in 3, one N₃⁻ and one H₂O molecule are coordinated to the Mn(II) centre. Complex 4 possesses a hexanuclear structure, in which two trinuclear Ni^II₂Mn^II units are connected via a single μ_1,3-azido bridge. Variable temperature dc molar magnetic susceptibility measurements reveal that the two Mn(II) centres in complex 4 are antiferromagnetically coupled with an exchange coupling constant (J) of −3.73 cm⁻¹. Among the three heterometallic complexes, only 3 and 4 show bio-mimetic catalytic oxidase activities. For catecholase like activity, the turnover numbers (K_cat) of complexes 3 and 4 are 984 and 2081 h⁻¹, respectively, whereas for phenoxazinone synthase-like activity, the turnover numbers of complexes 3 and 4 are 6351 and 10 545 h⁻¹, respectively. The high catalytic oxidase activities of complexes 3 and 4 in contrast to the inactivity of complex 2 are attributed to the coordination of a water molecule to the Mn(II) centre in the former structures, which facilitates the binding of the substrate with the catalyst. Mass spectral analyses suggest the probable formation of a complex–substrate intermediate in these catalytic reactions and cyclic voltammetry measurements show that Ni(II) is reducible to Ni(I).