For manganese mononitride (MnN), the total energy versus lattice constant is obtained using the spin density functional theory. Instead of the tetragonally distorted NaCl structure, we study the zinc blende and wurtzite structures in which AlN, GaN, and InN crystallize. The ground state with nonmagnetic, antiferromagnetic (AFM), or ferromagnetic (FM) arrangement of spins depends on the polymorph of MnN and on the lattice constant. At equilibrium lattice constants, in zinc blende it is AFM in  direction, and in wurtzite it is FM. The zinc blende polytype of MnN under hydrostatic pressure at the InN lattice constant presents FM ground state. For the wurtzite polytype at the GaN and AlN lattice constants, the AFM is the ground state, but goes back to a FM ground state for the InN lattice constants. For both structures, the system presents a half-metallic state at InN lattice constants (with a total magnetic moment of 4 μB per Mn atom) instead of the metallic state obtained for smaller lattice constants. Results indicate that the FM or the AFM state of Ga1−xMnxN and In1−xMnxN may be related to, relaxed, or strained, MnN incorporations or Mn-rich composition fluctuations.