The Parliamentary Borough of Colchester had sent two members to Parliament since the Model Parliament of 1295. In 1885, representation was reduced to one, being one of 36 English boroughs and three Irish boroughs to which this occurred under the Redistribution of Seats Act 1885. Under the Representation of the People Act 1918, the Parliamentary Borough was abolished and replaced with a Division of the County of Essex (later a County Constituency).
The constituency remained virtually unchanged until it was briefly abolished for the 1983 general election following the Third Periodic Review of Westminster Constituencies, but re-established for the 1997 general election as a Borough Constituency by the Fourth Review.
1918–1950: The Borough of Colchester, and the Rural District of Lexden and Winstree except the detached part of the parish of Inworth which was wholly surrounded by the parishes of Great Braxted and Kelvedon.
The area comprising the Rural District of Lexden and Winstree had largely been part of the Harwich Division.
1950–1983: The Borough of Colchester, the Urban District of West Mersea, and the Rural District of Lexden and Winstree.
No changes (the Urban District of West Mersea had been formed as a separate local authority in 1926).
For the 1983 general election, the constituency was abolished, with the northern parts (comprising the majority) forming the bulk of the new constituency of North Colchester. Southern areas were included in the new constituency of South Colchester and Maldon.
1997–2010: The Borough of Colchester wards of Berechurch, Castle, Harbour, Lexden, Mile End, New Town, Prettygate, St Andrew's, St Anne's, St John's, St Mary's, Shrub End, and Stanway.
Re-established from parts of the abolished constituencies of South Colchester and Maldon (Berechurch, Harbour, New Town, Prettygate and Shrub End wards) and North Colchester (remaining wards).
2010–present: The Borough of Colchester wards of Berechurch, Castle, Christ Church, Harbour, Highwoods, Lexden, Mile End, New Town, Prettygate, St Andrew's, St Anne's, St John's, and Shrub End.
Local authority wards redistributed. Minor reduction in electorate, with Stanway ward being included in the new constituency of Witham.
The seat has one of Britain's largest residential military populations. The non-military vote in Colchester swung further in favour of the Liberal Democrats since 1997 when Bob Russell stood. He was elected for the party with a small majority. Russell increased his votes and percentage share in three elections. In the 2010 election this was the only non-Conservative seat in Essex. Russell was defeated in the 2015 general election by Conservative Will Quince, by an 11.5% majority. In the 2017 election Quince was re-elected by a slightly decreased margin by percentage (10.6%), with Labour moving into 2nd place after a substantial increase in their vote, meaning this seat is now much more a contest between Labour and the Conservatives for the first time in many years. In the 2019 election, Quince increased his majority to 9,423 or 17.6% in percentage terms.
^Webster and Rebow were re-elected in 1714, but on petition the result was reversed and Gore declared to have been duly elected instead, following a dispute over whether foreigners could be made freemen of the borough and thereby acquire voting rights
^Webster was re-elected in 1710, but on petition the result was reversed and Gore and Corsellis declared to have been duly elected instead, following a further dispute over foreign freemen's voting rights
^At the election of 1741, Olmius and Martin were returned as elected, but on petition their election was declared void and their opponents, Savill and Gray, declared elected in their place
^At the election of 1754, Gray was re- elected, but on petition his election was declared void and his opponent, Rebow, declared elected in his place
^ abOn petition, Potter's election was declared void on the grounds of defective qualification and his opponent, Affleck, declared duly elected
Another General Election was required to take place before the end of 1940. The political parties had been making preparations for an election to take place from 1939 and by the end of this year, the following candidates had been selected;
Another General Election was required to take place before the end of 1915. The political parties had been making preparations for an election to take place and by July 1914, the following candidates had been selected;