The area which ESSEX now occupies was ruled pre-Roman settlement by the Celtic Trinovantes tribe. A dispute between them and another tribe was used as an excuse for a Roman invasion in 55 BC, and they allied with Rome when Claudius returned in 49 AD. This led to Camulodunum (Colchester) transferring from the Trinovantes to the Roman Empire as the capital of Roman Britain. The Trinovantes later fought with the Iceni tribe against Roman rule; perhaps they regretted collaborating with Claudius.

Following the Norman conquest the Saxon kingdom formed the basis of a county in 1139 under the first Earl of Essex, Geoffrey de Mandeville. As a county Essex had administrative, political and legal functions.

Much of the development of the county was caused by the railway. By 1843 the Eastern Counties Railway had connected Bishopsgate station with Brentwood and Colchester, in 1856, they opened a branch to Loughton (later extended to Ongar) and by 1884 the London, Tilbury and Southend Railway had connected Fenchurch Street railway station in the City of London to Grays, Tilbury, Southend-on-Sea and Shoeburyness. Some of the railways were built primarily to transport goods but some (e.g. the Loughton branch) were deliberately planned to cater for commuter traffic; they unintentially created the holiday resorts of Southend, Clacton and Frinton-on-Sea.

County councils were created in England in 1889. Essex County Council was based in Chelmsford, although it met in London until 1938. Its control did not cover the entire county. The London suburb of West Ham and later East Ham and the resort of Southend-on-Sea became county boroughs independent of county council control.