Abstract The influence of 120 MeV Ag ion irradiation on the structural and magnetic properties of Ni–Mn–Sn ferromagnetic shape memory alloy thin film is investigated. X-ray diffraction data confirms the phase transformation from martensite to austenite phase at a fluence of 1 × 10 13 ions/cm 2 , which is further supported by the change in surface morphology of the film with increasing fluence as evidenced by field emission scanning electron microscopy. Thermo-magnetic measurements reveal the increase in magnetization and decrease in phase transformation temperatures with increasing fluence. The maximum value of magnetization is ∼2.9 × 10 5 Amp/meter for the film irradiated at a fluence of 1 × 10 13 ions/cm 2 . The results are explained on the basis of thermal spike model considering the core and halo regions of ion tracks in FSMA materials.