The coordination-driven self-assembly of two polydentate linear Schiff base ligands (either N,N-bis(2-hydroxy-3-methoxy-benzyliden)-1,4-diaminobenzene, L2, or N,N-bis(2-hydroxy-3-methoxy-benzyliden)-1,5-diaminonaphthalene, L3) with two transition metal ions (MII = NiII or CoII) and two lanthanide ions (LnIII = GdIII or DyIII) afforded seven linear M2Ln2 complexes of formula [Ni2Ln2(L2)2(CH3CN)3(H2O)(NO3)6](CH3CN)2(H2O) (LnIII = Gd 1 and Dy 2) and [M2Ln2(L3)2(CH3CN)4(NO3)6](CH3CN)x (M = NiII, CoII; Ln = DyIII, GdIII,YIII; x = 0–4) (3–7). Within the tetranuclear units of these complexes, two ligands coordinate through the N,Ophenoxide donor sets to two M(II) ions, giving rise to M2 metallacycles. In the case of complexes 1–2, the Ni2-metallacycle contains 14-members, where the NiII ions are bridged by para-phenylenediimine groups. In complexes 3–7, the M2-metallacycle consists of 18-members, where the transition metal ions are linked by naphthalenediimine bridging groups. At both sides of these metallacycles, the MII ions are connected to LnIII ions through phenoxido bridging groups. The analysis of the dc and ac magnetic properties of these complexes reveals that: (i) all the compounds exhibit weak ferromagnetic exchange interactions between the MII and LnIII ions through the bis(phenoxido) bridging groups and weak antiferromagnetic MII–MII interactions transmitted by the acenediimine bridging groups; (ii) DFT calculations not only support the nature and magnitude of the magnetic exchange interactions, but also the polarization mechanism for the MII–MII interactions through the acene bridging legends; (iii) the antiferromagnetic interaction for 1 is stronger than for 3, which can be justified by the longer intermetallic Ni⋯Ni distance and α,α′-substitution for the latter; (iv) complexes 2 and 4 show slow relaxation of the magnetization below 5 K at zero static magnetic field with Ueff/kB values of 19 K and 15.9 K respectively, the higher Ueff/kB value corresponding to the stronger JDyGd coupling constant; (v) the change of −ΔSm for the M2Gd2 complexes 1, 3 and 6 has been analyzed by taking into account the values of their J and J1 magnetic exchange interactions and single-ion anisotropies.