Manganese antimonide Mn_2-xM_xSb, where M is a 3d transition metal, is a prominent binary material due to its high Curie temperature and magnetocaloric properties accompanying the M-induced first-order phase transition for various compositions. In this work we employed modern ab initio approach to analyze the magnetic ground state and electronic structure of Mn₂Sb for various types of long-range ordering. Mn₂Sb has a tetragonal type crystal structure with the space group P4/nmm. The unit cell contains two magnetic types of sites Mn1 and Mn2, tetrahedrally and octahedrally surrounded by Sb atoms, respectively. The structure is stable over a wide temperature range, with the magnetic structure passing through a series of phase transitions. The magnetic phase transition in Mn₂Sb occurs at the Curie temperature about 550 K. Below this temperature, a ferrimagnetic ordering with unequal magnetic moments of different types of manganese ions is realized in the compound. The ferrimagnetic structure is formed by an antiparallel arrangement of magnetic moments with different magnitude of the Mn magnetic moments and found to have the lowest total energy. We obtained the Mn magnetic moments as approximately -3.5 and 2.3 Bohr magnetons in a good agreement with experimental data. In the electronic structure of Mn₂Sb, it was found to possess the semi-metallic properties with a gap in the minority spin projection.