Magnetic field-induced transformation from paramagnetic austenite to ferromagnetic martensite in an Fe-21Ni-4Mn (wt%) alloy with dual martensitic transformation kinetics has been studied by magnetization measurement and optical microscopy, applying a pulsed ultra-high magnetic field. As a result, the following were found. A magnetic field higher than a critical one is needed to induce the martensitic transformation above Ms. The critical magnetic field increases with increasing temperature, and when plotted against the temperature difference (ΔT) from Ms, it lies on a straight line not passing through the origin. This result and thermodynamical analysis suggest that pulsed magnetic field strongly promotes the athermal martensitic transformation and restrains the isothemal one. The influence of magnetic field on martensitic transformation in the present Fe-Ni-Mn alloy is mainly due to Zeeman effect. The entropy change for athermal transformation at Ms, ΔSΔ_Ms^at is obtained to be 4.13 J/mol·K. The amount of magnetic field-induced martensites increases linearly with the maximum strength of pulsed magnetic field. Lath, plate and butterfly martensites are formed under magnetic field.