1955–1974: The county of Midlothian, including all the burghs situated therein, except the county of the city of Edinburgh and the burgh of Musselburgh.
1974–1983: As above.
1983–1997: Midlothian District.
1997–2005: The Midlothian District electoral wards of Bonnyrigg/Newtongrange, Dalkeith, Loanhead, and Mayfield/Gorebridge.
2005–present: The area of the Midlothian Council. When first formed, the constituency covered the whole of the traditional county of Midlothian, apart from
Edinburgh, split into multiple separate constituencies, and
Musselburgh, which was included in
Edinburgh East (UK Parliament constituency). In the 1983 reforms the constituency was significantly reduced from over 100,000 voters to 61,143, with large numbers of voters moved to the new seat of
Livingston, and smaller numbers to
Edinburgh Pentlands,
East Lothian and
Tweeddale, Ettrick and Lauderdale. The 1997 reforms moved
Penicuik into Tweeddale, Ettrick and Lauderdale, with the rest of the council area remaining in the seat; it returned in 2005, when the constituency was expanded again to cover the entire
Midlothian Council area. Under the
2023 review of Westminster constituencies, which came into effect for the
2024 general election, the boundaries were unchanged. == Members of Parliament ==