This county constituency consisted of the historic county of Middlesex, in south-eastern England, comprising Spelthorne, Colnbrook, and Potters Bar in other modern counties, together with the north, west, and north-west sectors of the present-day Greater London. 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) until 1885. The place of election for the county was at Brentford.
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 electors in the 1715–54 period. Namier and Brook suggested there were about 3,500 county voters in 1754–90. The number had reached about 6,000 by 1790–1820, according to Thorne.
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 1868 two further metropolitan Boroughs were each granted two members of parliament: Chelsea and Hackney. 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, relating to property situated in an area not otherwise represented, conferred the right to vote in the county elections.
In 1885 the county constituency was split into a number of single-member boroughs in the more urban south-east and seven county divisions elsewhere, while the City of London was reduced to two members.
In 1889 the borough constituencies that comprised the eastern part of the historic county became part of the new administrative county of London. The seven county divisions in the fringe part of the historic county, in what became the administrative county of Middlesex, were Brentford, Ealing, Enfield, Harrow, Hornsey, Tottenham and Uxbridge. None of these formed parliamentary boroughs and were in the administrative county of Middlesex until it was disbanded 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. It should be noted that 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).
Some of the members elected during this period have been identified, but this list does not include Parliaments where no member has been identified before the reign of King Henry VIII. In the list (as opposed to the table below) the year given is for the first meeting of the Parliament, with the month added where there was more than one Parliament in the year. If a second year is given this is a date of dissolution. Early Parliaments usually only existed for a few days or weeks, so dissolutions in the same year as the first meeting are not recorded in this list If a specific date of election is known this is recorded in italic brackets. The Roman numerals in brackets, following some names, are those used to distinguish different politicians of the same name in 'The House of Commons' 1509-1558 and 1558-1603.
In this period, Parliament was not an institution with a regular pattern of elections and sittings. Therefore, a separate entry is made for each Parliament, even if the same Knight of the Shire served in successive Parliaments.
List of known Knights of the Shire before 1509
Table of Knights of the Shire 1509-1660
Notes:-a Speaker of the House of Commons.
b Wroth ceased to be an MP after 11 May 1535. It is unknown if there was a by-election.
c Hawkes ceased to be MP by May/June 1532. It is unknown if there was a by-election.
d Hughes ceased to be an MP after January/April 1543. It is unknown if there was a by-election.
e In theory the Long Parliament existed throughout the 1640-1660 term, as it could not be lawfully dissolved without its own consent which was not given until 1660. In practice all or part of the membership of the House of Commons were not permitted to sit for lengthy periods. Other bodies considered to be Parliaments existed within parts of the term of the Long Parliament.
f Francklyn died and a by-election was held.
g In December 1648, Gilbert was excluded from Parliament in Pride's Purge and the seat was left vacant.
h Spencer is not recorded as having sat after Pride's Purge in December 1648.
Table of Members of the Commonwealth Parliaments 1653-1659
The County had three nominated members in the Barebones Parliament, four representatives in the First and Second and the usual two in the Third of the Protectorate Parliaments
Notes:-a Smithson, not the same man as the former member of the same name, changed his surname to Percy before the 1741 general election.
b Byng received the courtesy title of Viscount Enfield in 1860.
Leigh Rayment's Historical List of MPs – Constituencies beginning with "M" (part 2)
In multi-member elections the bloc voting system was used. Voters could cast a vote for one or two candidates, as they chose. The leading candidates with the largest number of votes were elected.
In by-elections, to fill a single seat, the first past the post system applied.
After 1832, when registration of voters was introduced, a turnout figure is given for contested elections. In two-member elections, when the exact number of participating voters is unknown, this is calculated by dividing the number of votes by two. To the extent that electors did not use both their votes this will be an underestimate of turnout.
Where a party had more than one candidate in one or both of a pair of successive elections change is calculated for each individual candidate, otherwise change is based on the party vote.
Candidates for whom no party has been identified are classified as Non Partisan. The candidate might have been associated with a party or faction in Parliament or consider himself to belong to a particular political tradition. Political parties before the nineteenth century were not as cohesive or organised as they later became. Contemporary commentators (even the reputed leaders of parties or factions) in the eighteenth century did not necessarily agree who the party supporters were. The traditional parties, which had arisen in the late seventeenth century, became increasingly irrelevant to politics in the eighteenth century (particularly after 1760), although for some contests in some constituencies party labels were still used. It was only towards the end of the century that party labels began to acquire some meaning again, although this process was by no means complete for several more generations.
Sources: The results for elections 1660-1790 were taken from the History of Parliament Trust publications. The results are based on Stooks Smith from 1790 until the 1832 general election and Craig from 1832. Where Stooks Smith gives additional information after 1832 this is indicated in a note.Note (1660) vote totals unavailable
Note (1661) vote totals unavailable
Note (1679): Roberts was not the same man as the 1660 candidate of the same name.
Note (1679): Smyth is referred to as Smith in House of Commons 1660-1690, but Smyth seems to be correct from Leigh Rayment's list of baronets.
Expulsion from the House of Peyton 
Note (1685) vote totals unavailable. Smyth is referred to as Smith in House of Commons 1660-1690, but Smyth seems to be correct from Leigh Rayment's list of baronets.
Note (1689) vote totals unavailable
Choice of Russell to sit for Cambridgeshire
Death of Wolstenholme
Death of Child
Smithson (not the same person as the former MP of the same name) subsequently changed his surname to Percy
Creation of Pulteney as 1st Earl of Bath
Succession of Percy as 2nd Earl of Northumberland
Appointment of Cooke as Joint Paymaster of the Forces
Note (1768): Stooks Smith attributes 1,292 votes to Wilkes. Stooks Smith does not give candidates party labels in Middlesex until after this election.
Death of Cooke
Note (1768): Poll 6 days (Source: Stooks Smith)
Expulsion from the House of Wilkes, declared incapable of being elected 3 February 1769
Expulsion from the House of Wilkes, election declared void
Expulsion from the House of Wilkes, election declared void 17 March 1769
Election return of Wilkes amended to Luttrell by Parliament on 14 April 1769 and Luttrell seated as the MP 15 April 1769
Death of Glynn
Note (1790): The George Byng who contested Middlesex elections from this year is a different person from the one who stood previously
Note (1802): Poll 15 days (Source: Stooks Smith)
Election of Burdett declared void 9 July 1804
Note (1804): Poll 15 days (Source: Stooks Smith)
Election of Mainwearing challenged by a petition of Burdett. Mainwaring unseated and Sir Francis Burdett, Bt seated on 5 March 1805. (Source: The Times (of London), edition of 6 March 1805)
Election of Burdett challenged by a petition of Mainwearing. Burdett unseated and George Boulton Mainwaring seated with effect from 10 February 1806. (Source: The Times (of London), edition of 10 February 1806)
Note (1806): Poll 15 days (Source: Stooks Smith)
Note (1820): Poll 12 days (Source: Stooks Smith)
Note (1832): 5,132 voted. Hume was classified as a Radical candidate. (Source: Stooks Smith).
Note 1 (1835): 6,046 voted. Hume was classified as a Radical candidate. (Source: Stooks Smith).
Note 2 (1835): The Thomas Wood who contested Middlesex elections from this year is a different person from the one who was elected in 1779
Note (1837): 9,260 voted. Hume was classified as a Radical candidate. (Source: Stooks Smith).
Death of Byng
Creation of Grosvenor as 1st Baron Ebury
Byng became known by the courtesy title of Viscount Enfield when his father became 2nd Earl of Strafford in 1860
Death of Hanbury
Appointment of Hamilton as Vice-President of the Privy Council Committee on Education
Appointment of Hamilton as First Lord of the Admiralty
Constituency divided in the 1885 redistribution