<p>Half-Heusler compounds are promising thermoelectric materials for high-temperature applications, yet their performance is limited by high lattice thermal conductivity. Here, we present an alternative approach to synthesize ZrNiSn-based half-Heusler compounds with hierarchical architectures across multiple length scales. By utilizing short-duration mechanical alloying to produce nonequilibrium precursors, followed by reactive sintering, we enable precise control over phase composition and microstructural features. This approach results in multi-scale architectures comprising interstitial defects, grain boundaries, nanoprecipitates, and pores, enabling strong phonon scattering. The optimized Zr_0.75Hf_0.25NiSn_0.99Sb_0.01 alloy exhibits a lattice thermal conductivity as low as 1.9 W m⁻¹ K⁻¹ and a high power factor of 50 µW cm⁻¹ K⁻², yielding an impressive dimensionless figure of merit (<i>zT</i>) of 1.33 at 873 K. This performance surpasses that of ZrNiSn-based compounds synthesized via conventional methods such as arc melting and solid-state reaction. Our method, distinguished from conventional melting synthesis approaches through its simplicity, cost-effectiveness, and scalability, provides a versatile framework for achieving efficient hierarchical phonon scattering while preserving high carrier mobility in half-Heusler compounds and highlights the potential of reactive sintering for advancing thermoelectric materials.</p>