mid-14c., "a shire, a definite division of a country or state for political and administrative purposes," from Anglo-French counte, from Late Latin comitatus "jurisdiction of a count," from Latin comes (see count (n.1)). It replaced Old English scir "shire."
From late 14c. as "the domain of a count or earl." County palatine, one distinguished by special privileges (Lancaster, Chester, Durham) is from mid-15c. County seat "seat of the government of a county" is by 1848, American English.