The borough has sent representatives to Parliament since the Model Parliament of 1295: two members were sent until 1885, when 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.
1918-1950: The Municipal Borough of Colchester, and the Rural District of Lexden and Winstree except the detached part of the civil parish of Inworth which was wholly surrounded by the civil parishes of Great Braxted and Kelvedon.
1950-1983: The Municipal Borough of Colchester, the Urban District of West Mersea, and the Rural District of Lexden and Winstree.
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.
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.
Once the basis for one or two semi-rural seats, the modern-day Colchester constituency is a compact, urban core, containing the town centre and surrounding neighbourhoods.
The seat has one of Britain's largest residential military populations. The non-military vote in Colchester swang 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 decreased margin by percentage (10.6%) yet increased votes (5,646 votes more) in an election where turnout nationally had greatly increased — the seat saw a 19.1% increase in the Labour vote in 2017, a Labour candidate had last achieved second place in the area in 1979, with the seat now becoming a somewhat marginal contest between them and the Conservatives.
^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 the July 1914, the following candidates had been selected;
^"Essex". Bury and Norwich Post. 21 February 1888. p. 8. Retrieved 25 November 2017 – via British Newspaper Archive. (Subscription required (help)). Cite uses deprecated parameter |subscription= (help)