Rechargeable magnesium batteries offer a compelling pathway toward sustainable energy storage, yet realizing this vision demands scalable electrolytes formulated with widely accessible salts, such as magnesium bis(trifluoromethanesulfonyl)imide (Mg(TFSI)₂). Progress in Mg(TFSI)₂-based electrolytes has been severely hindered by low Mg stripping/plating efficiency, arising from sluggish Mg²⁺ desolvation kinetics and uncontrolled interfacial reactions. Herein, we adopt a descriptor-informed molecular screening approach and identify 4-bromo-N, N-dimethylaniline (BrNNA) as an interface-active electrolyte component that enables fast Mg stripping/plating kinetics in Mg(TFSI)₂-based electrolytes. Recognizing their relevance to interface modulation, dipole moment, lowest unoccupied molecular orbital energy, and donor number are employed as quantifiable molecular descriptors to screen a series of substituted aniline derivatives, from which BrNNA emerges as the optimal candidate. The optimized BrNNA-containing electrolyte supports immediate and stable Mg electrochemistry without activation, sustaining over 2400 h at 0.1 mA cm⁻² in symmetric cells and achieving cumulative areal capacities up to 5 Ah cm⁻² at 10 mA cm⁻². We reveal that BrNNA participates in Mg²⁺ solvation while preferentially adsorbing onto the Mg surface, lowering the Mg²⁺ desolvation barrier and directing the formation of a thin Br-containing interface that facilitates interfacial Mg²⁺ transport. Moreover, the electrolyte has been validated in full Mg cells using various cathode materials.