Due to the “impurity bands” caused by interstitial Ni defects, n-type MNiSn (M=Ti, Zr or Hf)-based half-Heusler compounds have demonstrated excellent thermoelectric properties, but the p-type counterparts suffer from a figure-of-merit (zT) that is significantly lower than the predictions based on ab initial calculations. Herein, we develop a new p-type MNiSn compound with a high zT of 0.6 at 973 K, which exceeds the state-of-the-art by ~100%. Upon Co doping in HfNiSn, the electrical transports were shifted from n- to p-type with an enhanced Seebeck coefficient. The optimized carrier concentration yields a maximum power factor of ~2.2 mWm^-1K^-2 at 973 K. In combination with further phonon scattering through Zr alloying, a minimum lattice thermal conductivity of 1.9 Wm^-1K^-1 is obtained. This work indicates that p-type half-Heusler MNiSn is a promising candidate for thermoelectric generators, which will motivate further research of thermoelectric materials and devices based on both n- and p-type MNiSn legs. Our strategy provides new insights into the intrinsic atomic disorder degree of the half-Heusler system, facilitating further optimization of electronic and thermal properties.