We present a first-principles study of the electronic and magnetic properties of cubic hafnium dioxide stabilized by Mn. We find this material to be ferromagnetic and half-metallic, with the Mn-impurity electronic states laying in the band gap of hafnia for a wide range of manganese concentration. Our ab initio calculations, within the local spin-density approximation, demonstrate that Mn-doped hafnia may be ferromagnetic at 700 K while its high- T_C-ferromagnetism is robust to the oxygen vacancy defects and to how the Mn impurities are distributed over the cation sublattice.