Ni–Mn–Ga is one of the major magnetic materials in Heusler family due to its interesting magnetic and structural properties. In this study, the epitaxial 200 nm thin films are prepared by magnetron co-sputtering from a NiMnGa alloy target and an elemental Fe target on MgO substrates. The epitaxial growth mechanism is pursued and controlled by ex-situ X-ray refractometry (XRR) and X-ray diffraction (XRD) measurements. On account of analysing the physical parameters, temperature-dependent X-ray diffraction (XRD), transmission electron (TEM), scanning electron microscopy (SEM), temperature dependence of resistivity (ρ(T)), atomic force microscopy (AFM) and magnetisation (MOKE) measurements are performed systematically for different Fe contents. Structural and magnetic measurements show that the NiMnGa parent thin film transitions could be adjusted from martensitic to austenitic phases by adding Fe element from high temperature to the vicinity of room temperature. Measurements are carried out to get a further structural characterisation of 14M martensite which was found to consist of nano-twinned structures. The epitaxial growth and the stable coexisting austenite and martensitic phases for the thin film of the NiMnGa chemical composition are shown near room temperature. Due to this feature, magnetic thin films could increase the likelihood of possible room temperature using thin film applications.