Influence of disorder, antisite defects, martensite transition and compositional variation on the magnetic properties and electronic structure of Mn 2 NiGa and Mn 1+ x Ni 2− x Ga magnetic shape memory alloys have been studied by using full potential spin-polarized scalar relativistic Korringa–Kohn–Rostocker (FP-SPRKKR) method. Mn 2 NiGa is ferrimagnetic and its total spin moment increases when disorder in the occupancy of Mn Ni (Mn atom in Ni position) is considered. The moment further increases when Mn–Ga antisite defect [1] is included in the calculation. A reasonable estimate of T C for Mn 2 NiGa is obtained from the exchange parameters for the disordered structure. Disorder influences the electronic structure of Mn 2 NiGa through overall broadening of the density of states and a decrease in the exchange splitting. Inclusion of antisite defects marginally broaden the minority spin partial DOS (PDOS), while the majority spin PDOS is hardly affected. For Mn 1+ x Ni 2− x Ga where 1⩾ x⩾ 0, as x decreases, Mn Mn moment increases while Mn Ni moment decreases in both austenite and martensite phases. For x⩾ 0.25, the total moment of the martensite phase is smaller compared to the austenite phase, which indicates possible occurrence of inverse magnetocaloric effect. We find that the redistribution of Ni 3d-Mn Ni 3d minority spin electron states close to the Fermi level is primarily responsible for the stability of the martensite phase in Mn–Ni–Ga.