The Zr distribution in a commercial Mg-30Zr master alloy was modified by tungsten inert gas arc re-melting with ultra-high frequency pulses (UHFP-TIGR) leading to the most substantial grain size reduction using a Mg-Zr master alloy. The results show that the Zr particle size is well refined, and higher contents of soluble Zr (Zr_s) and nanoscale (∼10 nm) Zr particles (Zr_np) can be obtained. Zr_s provides considerable constitutional supercooling (CS) for the nucleation of α-Mg grains on Zr_p (∼100 nm particles). An exceptional improvement in the grain refinement efficacy (54%) of Zr master alloys is achieved with a holding time of 5 min. Consequently, a new approach to the design and fabrication of high-efficiency grain refiners is proposed.