Recently synthesized Ba ₄ NbMn ₃ O ₁₂ belong to cluster magnets that are systems with tightly bound groups of magnetic ions, in this case Mn ₃ trimers. Such magnetic clusters can often be described by molecular orbitals, however strong electron correlations may invalidate this description. To understand the electronic and magnetic state of Ba ₄ NbMn ₃ O ₁₂ we carried out ab initio calculations and show that this system is better described not in molecular orbitals picture, but as a system with electrons localized on the Mn ions, with strong intracluster and weaker inter-cluster exchange. The calculated spin of the Mn ₃ trimer is S = 2, in agreement with the experiment. The predicted magnetic structure of Ba ₄ NbMn ₃ O ₁₂ is that of ferromagnetic layers of Mn ₃ trimers, stacked antiferromagnetically.