Effect of the temperature and magnetic field induced martensitic transformation (MT) on the electronic structure and some physical properties of bulk Fe45.2Mn25.9Ga28.9 Heusler alloy has been investigated. According to the experimental results of DSC, magnetic and transport measurements direct and reverse martensitic transformation without external magnetic field takes place within 194  T  328 K temperature range with a hysteresis up to ΔT ~100 K defined as ΔT = A_f,s - M_s,f , where A_f,s and M_s,f are the critical temperatures of direct and reverse martensitic transformation. External magnetic field of μ₀H = 5 T causes a hightemperature shift of MT temperatures. MT from parent austenite L21 phase to martensitic tetragonally distorted L21 one (i. e. to L10) causes significant changes in the electronic structure of alloy, a drastic increase in alloy magnetization, a decrease in the alloy resistivity, and a reversal of sign of the temperature coefficient of resistivity from negative to positive. At the same time experimentally determined optical properties of Fe_45.6Mn_25.9Ga_28.9 Heusler alloy in austenitic and martensitic states look visually rather similar being noticeable different in microscopic nature as can be concluded from first-principle calculations. Experimentally observed changes in the physical properties of the alloy are discussed in terms of the electronic structures of an austenite and martensite phases.