The electronic and magnetic structures of the tetragonal and hexagonal MnFeAs were examined using density functional theory to understand the reported magnetic orderings and structural change induced by high-pressure synthesis. The reported magnetic ground states were confirmed using VASP total energy calculations. Effective exchange parameters for metal–metal contacts obtained from SPRKKR calculations indicate indirect exchange couplings are dominant in tetragonal MnFeAs. Weak direct exchange couplings for adjacent Fe–Fe and Fe–Mn contacts cause the coexistence of several low-energy magnetic structures in tetragonal MnFeAs and result in a near zero magnetic moment on the Fe atoms. On the other hand, the nearest-neighbor Fe–Fe and Fe–Mn interactions in hexagonal MnFeAs are a combination of direct and indirect exchange couplings. In addition, indirect exchange couplings in tetragonal MnFeAs are rationalized by both RKKY and superexchange mechanisms. Finally, to probe the high-pressure-induced phase transition, total energy changes with the change of volume was studied on both tetragonal and hexagonal MnFeAs.