We developed a liquid chromatography-mass spectrometry-based method for the simultaneous quantification of proteins from all 21 allergenic foods regulated by the Korea Ministry of Food and Drug Safety. First, a systematic process was employed to discover, verify, and validate allergenic food protein marker peptides, resulting in 62 validated marker peptides. All marker peptides showed excellent sensitivity. At least one marker peptide for each allergenic food demonstrated limits of detection at sub-ppm levels, except pine nut (3-6 ppm). Second, a LC-MS multiple reaction monitoring method was optimized to simultaneously identify and quantify all marker peptides in a single LC-MS analysis. Third, application of our optimized method to representative commercial processed foods as a pilot-level screening suggested the potential occurrence of undeclared allergenic food contaminations. These findings underscore the need for broader and more accurate allergenic food analysis, and for stricter allergenic food-labeling policies to protect consumers with food allergies.