Two isostructural heterometallic trinuclear oxalato-bridged complexes of formula C4[MCr2(ox)6(H2O)2]·nH2O (C+ = 4-aminopyridinium; ox2− = oxalate dianion; M2+ = Mn2+, n = 3, 1; M2+ = Co2+, n = 3.25, 2) have been synthesized by using direct self-assembly methods combining C3[Cr(ox)3] and the chloride salts of the corresponding metal ion. The crystal structures of both compounds have been resolved by single-crystal X-ray diffraction. They crystallize in the C2/c space group [a = 11.5113(15) Å, b = 20.250(3) Å, c = 21.810(4) Å, β = 100.447(10)°, V = 5161.6(3) Å3, and Z = 4 for 1, and a = 11.4334(16) Å, b = 20.243(2) Å, c = 21.805(3) Å, β = 101.113(9)°, V = 4951.9(11) Å3, and Z = 4 for 2]. The structures of 1 and 2 consist of discrete linear [MCr2(ox)6]4− bimetallic trinuclear units, pyridinium cations and crystallization water molecules. The linear trinuclear unit is built from a central trans-diaquametal(II), linked to two Cr(ox)3]3− entities by oxalate bridges. One of the oxalate ions is coordinated to the central metal ion whereas the other two oxalate ligands are non-bridging. In the crystal, intermolecular hydrogen bonds involving oxalate ligands, water molecules and pyridinium cations, build a complex three-dimensional network. Variable-temperature magnetic susceptibility measurements for 1 and 2 indicate a weak ferromagnetic interaction (J = +1.16 and +2.62/+2.70 cm−1 for 1 and 2, respectively) between the two terminal CrIII (SCr = 3/2) and the central high-spin MnII (SMn = 5/2) and CoII (SCo = 3/2) ions. The nature and the amplitude of the exchange interaction are rationalized using DFT calculations and orbital interpretations.