This work reports the syntheses, crystal structures, magnetic properties and DFT calculations of six novel polynuclear NiII compounds [Ni2(μ-HL1)2(μ-N3)]N3·(CH3OH)2·2H2O (1), [Ni2(μ-HL1)2(μ-N3)][Na2Ni2(μ-L1)2(μ-N3)2(CH3OH)(N3)]·4CH3OH (2), [Ni4(μ-L1)2(μ-N3)4(CH3OH)2]·2CH3OH (3), [Na2Ni4(μ-L1)2(μ-OAc)2(μ-N3)4(CH3OH)4]·2CH3OH (4), [Ni4(μ-L1)2(μ-Cl)2(Cl)2] (5), and [Ni3(μ-L1)2(acac)2(H2O)2] (6), with uncommon structures and rare mixed-bridges between NiII ions, which were prepared from the versatile polytopic Mannich base ligand N,N′-dimethyl-N,N′-bis(2-hydroxy-3-methoxy-5-methylbenzyl)ethylenediamine (H2L1). The anionic coligand (azide, chloride, acetate and acetylacetonate) and reaction conditions play crucial roles in determining the final structure of these compounds and consequently their magnetic properties. Compound 1 contains a Ni2 cationic unit with rare di-μ-phenoxido/μ1,1-azide triple mixed bridges whereas complex 2 is made from the same Ni2 cationic unit as 1, cocrystallized with Na2Ni2 neutral units, in which double μ1,1,1-azide bridges connect the NiII and NaI anions. Complexes 3 and 4 are Ni4 complexes with defective dicubane and linear structures, respectively. Complex 3 has two distinct types of mixed bridges, μ-phenoxido/μ1,1,1-azido and μ1,1-azido/μ1,1,1-azido and a double di-μ1,1,1-azido bridge, the latter connecting the face-sharing NiII ions. In 4, rare μ-phenoxido/μ1,1-azide/syn–syn acetate triple mixed bridges connect central and terminal NiII atoms whereas a double μ1,1-azide planar bridging fragment links the central NiII ions. Complex 5 has a defective-dicubane structure with double μ-phenoxido/μ3-chloro mixed bridges and di-μ3-chloro bridges, whereas complex 6 has a bent structure with very uncommon single μ-phenoxido bridges. The analysis of the magnetic properties reveals that in complexes 1–4 all magnetic pathways transmit ferromagnetic interactions leading to S = 2 ground states for 1 and 2 and S = 4 ground states for 3 and 4. In complex 5, the double μ-phenoxido/μ3-chloro mixed bridges and di-μ3-chloro bridges mediate antiferro- and ferromagnetic interactions, respectively, giving rise to a S = 0 ground state. Complex 6 shows antiferromagnetic interactions between the NiII ions through single μ-phenoxido bridging groups, leading to an S = 1 ground state. DFT calculations on the X-ray structures and model compounds were performed to support the magneto-structural data of the above compounds.