Differential solvent extraction and phytochemical profiling of Chinse chive were employed to identify its principal PhIP-formation inhibitory constituents. Six compounds (mangiferin, isorhamnetin, luteolin, rosmarinic acid, 6-methylcoumarin, and cyanidin-3-glucoside) were further analyzed in a PhIP-producing chemical model to identify the dominant inhibitor. Its inhibitory mechanism was investigated by assessing the contribution of antioxidation and scavenging of key PhIP precursor/intermediate. No significant correlation was observed between PhIP inhibition rates and antioxidant activities. Further evaluation of the novel potent inhibitor mangiferin revealed a highly significant correlation between its dose-dependent inhibition of PhIP formation and phenylacetaldehyde scavenging. Finally, the proposed mechanism was corroborated through organic synthesis and structural elucidation of the mangiferin-phenylacetaldehyde adduct. This study has identified a potent novel inhibitor of the most abundant HA in heat-processed food and characterized its action mechanism. These findings may provide insight for future studies on mitigation of dietary exposure to toxic Maillard products by polyphenolic phytochemicals.