In this work, the electronic structure and optical behavior and the thermoelectric performance of the known HfNiSn compound have been studied under the substitution of Mn transition metal instead of Ni atoms. Necessary calculations are performed in the framework of DFT first principles studies by applying generalized gradient approximation (PBE-GGA) as well as solving Boltzmann’s semi-classical equations. The entering Mn leads to a change in the electronic structure of HfNiSn and the occurrence of half-metallic ferromagnetic behavior with 100% polarization at the Fermi level. The maximum ZT value obtained for HfMnSn shows that HfNiSn would be suitable for thermoelectric applications at room temperature, both in pure and Mn presence. The examination of optical parameters also indicates good absorption in the visible range for this compound in all cases.