Essex was originally formed as one of four administrative districts within the very brief reign of East Jersey in 1675, together with Bergen, Middlesex and Monmouth Counties. Essex County was formed within East Jersey on March 7, 1683.[1] The county was named in honor the English county of Kingdom of Essex, one of the historical kingdoms of England.

Portions of Essex were taken in 1741 and transferred to Somerset County. In 1837, Passaic County was formed from portions of Essex and Bergen County. In 1857, Union County was created from parts of Essex County.

— Wikipedia