ZrNiSn half-Heusler (HH) alloy has an interpenetrating face-centered-cubic crystal structure with four vacant positions in the unit cell that could accommodate foreign elements, playing an imperative role in the optimization of electronic transport by tuning the band structure and band gap, as well as reducing the phonon transport by interstitial point defect scattering. In the present study, filling the available voids at the 4d (¾, ¾, ¾) site in the HH unit cell of ZrNiSn with up to 4% Cu is realized. Excess Cu enhances the electronic transport by increasing the carrier density and also reduces the lattice thermal conductivity thus leading to a peak ZT value of 0.81 at 775 K for the composition ZrNiCu_0.02Sn. Further, the mechanical properties which are equally desirable for device fabrication also improved with Cu addition, confirming that this is an effective strategy to strengthen the mechanical properties of HH alloys.