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MnNiAs is half-Heusler structured and crystallizes in the cubic F-43m space group. The structure is zero-dimensional and consists of four arsenic molecules, four manganese molecules, and four nickel molecules.
MnNiAs crystallizes in the hexagonal P-62m space group. The structure is three-dimensional. Mn2+ is bonded to five As3- atoms to form MnAs5 square pyramids that share corners with ten equivalent MnAs5 square pyramids, corners with six equivalent NiAs4 tetrahedra, edges with six equivalent MnAs5 square pyramids, and edges with six equivalent NiAs4 tetrahedra. There are one shorter (2.54 Å) and four longer (2.61 Å) Mn–As bond lengths. Ni1+ is bonded to four As3- atoms to form NiAs4 tetrahedra that share corners with six equivalent MnAs5 square pyramids, corners with ten equivalent NiAs4 tetrahedra, edges with six equivalent MnAs5 square pyramids, and edges with two equivalent NiAs4 tetrahedra. There are two shorter (2.35 Å) and two longer (2.40 Å) Ni–As bond lengths. There are two inequivalent As3- sites. In the first As3- site, As3- is bonded in a distorted q6 geometry to three equivalent Mn2+ and six equivalent Ni1+ atoms. In the second As3- site, As3- is bonded in a 9-coordinate geometry to six equivalent Mn2+ and three equivalent Ni1+ atoms.