Middlesex was a constituency of the House of Commons of the Parliament of England, then of the Parliament of Great Britain from 1707 to 1800, then of the Parliament of the United Kingdom from 1801 until abolished in 1885. It returned two members per election by various voting systems including hustings.

This county constituency until 1832 covered all the historic county of Middlesex, in south-eastern England, comprising Spelthorne, Poyle, South Mimms and Potters Bar in other modern counties, together with the north, west, and north-west sectors of the present-day Greater London. Apart from the ability of some voters to participate in the borough franchises of the cities of London and Westminster (after dates of their inception, see top right or below), it gave rise to three more urban offshoot divisions in 1832, one of which was split in two at the next national review or reform, in 1868. Its southern boundary was the River Thames.

The county seat returned two Members of Parliament (sometimes referred to by the medieval term of knights of the shire). The place of election for the county was until 1700 at Hampstead Heath, thereafter at The Butts in the town centre of Brentford.^[1] Hustings were typically over a period of a fortnight when candidates set out their stall, and visible bribery had become not uncommon in closer contests around the country in such larger seats at the time, inspiring William Hogarth’s series of four pictures titled ‘Four Prints of An Election’ (when printed).^[1]

Until 1832 the county franchise was limited to forty shilling freeholders. The decrease in the value of money due to inflation and the expansion of the wealth and population as the urbanised area in the east around London and Westminster grew contributed to gradually expanding the electorate. The county was estimated by Henning to have about 1,660 voters in 1681. Sedgwick estimated about 3,000 in the 1715–54 period. Namier and Brook suggested there were about 3,500 in 1754–90. The number had reached about 6,000 by 1790–1820, according to Thorne. Close elections between popular candidates would therefore be expensive - the worth of being a local magistrate, major landowner or other dignitary carrying little weight among such a generally urban and numerous upper-middle class forming the bulk of the electorate.

For subsequent changes in the franchise see Reform Act 1832 and Reform Act 1867. From 1832 voters were registered; the size of the electorate is shown below.

The geographic county until 1885 also contained the borough constituencies of City of London (first recorded as having its extraordinary four members from 1298) and Westminster (enfranchised with two members from 1545). In 1832 three two-seat Boroughs were added (or enfranchised): Finsbury, Marylebone, and Tower Hamlets. In 1867 two new parliamentary boroughs each returning two MPs were constituted: 'Hackney' (St. Leonard's, Shoreditch, St Matthew's Bethnal Green and St John's Hackney) formerly represented in borough elections via Tower Hamlets and 'Chelsea' (parishes of Chelsea, Kensington, Hammersmith and Fulham).^[2] The single-member non-territorial University constituency of London University (1868–1950) was somewhat connected to the county by having most of its graduates eligible to vote.

Possession of a county electoral qualification, deriving from owning various types of property or having ecclesiastical 'offices' (controversially and sporadically defined) in an area not otherwise represented, conferred the right to vote in the county elections.

An 1885 redistribution of seats saw Middlesex and its early breakaway seats in and around the City reformed under the Redistribution of Seats Act 1885 reflecting the wider electorate of the Reform Act 1884 and need to 'liberate' boroughs, i.e. urban areas without properly apportioned representation:

• Constituencies in the urban south-east part that returned 18 MPs were replaced by 38 single-member seats.
• the City of London constituency (loosely considered with the county) was reduced from 4 to 2 members.
• the Middlesex constituency latterly covering the north, west and south-west of the county returning 2 MPs was replaced by 7 single-member seats.

Local government bodies

In 1889 the 40 urban constituencies that comprised the south-eastern part fell into (for local government) a County of London save for the much smaller City of London which remained a separate quasi-county and legal jurisdiction. The seven county divisions (constituencies) in the north and west of the historic county came under a new local government body, the administrative county of Middlesex. Both counties were also known by their governing bodies' name, County Councils (abbreviated to LCC and MCC). The seven successor seats were Brentford, Ealing, Enfield, Harrow, Hornsey, Tottenham and Uxbridge. These (and numerous later successor seats) had MCC local governance until its abolition in 1965.

Preliminary note: The English civil year started on Lady Day, 25 March, until 1752 (Scotland having changed to 1 January in 1600). The year used in the lists of Parliaments in this article have been converted to the new style where necessary. Old style dates for days between 1 January and 24 March actually referred to days after 31 December. No attempt has been made to compensate for the eleven days which did not occur in September 1752 in both England and Scotland as well as other British controlled territories (when the day after 2 September was 14 September), so as to bring the British Empire fully in line with the Gregorian calendar.

Constituency created (1265): See Montfort's Parliament for further details. Knights of the shire are known to have been summoned to most Parliaments from 1290 (19th Parliament of King Edward I of England) and to every one from 1320 (19th Parliament of King Edward II of England).

• List of former United Kingdom Parliament constituencies
• Unreformed House of Commons
• List of parliaments of England
• Duration of English, British and United Kingdom parliaments from 1660