The article deals with the problem of medieval custom as a legal and behavioral foundation in the process of precedent’s formation in the Common Law of England. Medieval custom is defined as a special algorithm of behavior based on a traditional character of its applying, which began its existence in “times out of mind” and had been living in the memory and everyday life of generations for a long period. Custom is also characterized as an invisible behavioral vector, legitimizing medieval everyday life. The most important features of the custom were as such: its immemorial character (origin from times out of mind), continuity of its support (often usage of the custom), its reliable character. Medieval custom might be considered as a base (a “nuclear”) of the very mechanism of a legal precedent’s formation; for the judges of an early (existing from 12th сentury) mobile royal courts with their system of writs had to rely on the local custom as the only reliable rule of behavior and legal traditions of a place. Legal custom was one of the most important sources of the Common Law of England. For instance, it is shown in the article, that royal power used and corrected so called the custom of “infangthief” (a word of Anglo-Saxon origin; means hanging a felon or a thief on the place of their crimes) connected with private seigniorial jurisdiction, and in the form of the royal Statute “Quo Warranto” (1290) transformed it in the instrument of the inquisition of the problems connected with the definition of property rights.