A heterometallic coordination polymer {[Cu(bpy)₃][Mn₂(C₂O₄)₃]·H₂O}_n (1; bpy = 2,2′-bipyridine) was synthesized using a building-block approach and characterized by IR spectroscopy, single-crystal X-ray diffraction, magnetization measurement, and X-band ESR spectroscopy both on a single crystal and a polycrystalline sample. The molecular structure of 1 is made of a three-dimensional (3D) anionic network [Mn₂(C₂O₄)₃]_n ²ⁿ⁻ and tris-chelated cations [Cu(bpy)₃]²⁺ occupying the vacancies of the framework. In compound 1 magnetic order is confirmed below 12.8 K – magnetization measurements reveal an antiferromagnetic-like network of canted Mn²⁺ spins with incorporated paramagnetic Cu²⁺ centres. The ESR spectroscopy distinctly shows the phase transition; above T ≈ 13 K, single isotropic Lorentzian lines of Mn²⁺ ions in the high spin state S = 5/2 were observed, while below this temperature, only characteristic Cu²⁺ signals from cations were detected. Thermal decomposition residues of 1 at different temperatures (800–1000 °C) were analyzed by powder X-ray diffraction; by heating the sample up to 1000 °C the spinel oxide CuMn₂O₄ [94.1(2) wt%] was formed. From the refined structural parameters, it could be seen that the obtained spinel is characterized by the inversion parameter δ ∼ 0.8, and therefore the structural formula at room temperature can be written as ^tet[Cu_0.17Mn_0.83]^oct[Mn_1.17Cu_0.83]O₄.