Personalized drug therapy aims to provide tailored treatment for individual patient. Mass spectrometry (MS) is revolutionarily involved in this area because MS is a rapid, customizable, cost‐effective, and easy to be used high‐throughput method with high sensitivity, specificity, and accuracy. It is driving the formation of a new field, MS‐based personalized drug therapy, which currently mainly includes five subfields: therapeutic drug monitoring (TDM), pharmacogenomics (PGx), pharmacomicrobiomics, pharmacoepigenomics, and immunopeptidomics. Gas chromatography‐MS (GC‐MS) and liquid chromatography‐MS (LC‐MS) are considered as the gold standard for TDM, which can be used to optimize drug dosage. Matrix‐assisted laser desorption ionization‐time of flight‐MS (MALDI‐TOF‐MS) significantly improves the capability of detecting biomacromolecule, and largely promotes the application of MS in PGx. It is becoming an indispensable tool for genotyping, which is used to discover and validate genetic biomarkers. In addition, MALDI‐TOF‐MS also plays important roles in identity of human microbiome whose diversity can explain interindividual differences of drug response. Pharmacoepigenetics is to study the role of epigenetic factors in individualized drug treatment. MS can be used to discover and validate pharmacoepigenetic markers (DNA methylation, histone modification, and noncoding RNA). For the emerging cancer immunotherapy, personalized cancer vaccine has effective immunotherapeutic activity in the clinic. MS‐based immunopeptidomics can effectively discover and screen neoantigens. This article systematically reviewed MS‐based personalized drug therapy in the above mentioned five subfields. © 2020 John Wiley & Sons Ltd. Mass Spec Rev