Peeblesshire was a Scottish county constituency of Great Britain and after 1801 the House of Commons of the Parliament of the United Kingdom (Westminster) from 1708 until 1868.
It elected one Member of Parliament (MP) by the first past the post voting system.
Article XII of the Union with Scotland Act 1706 (Act settling the Manner of electing the Representatives of Scotland), provided that one representative should be chosen for every shire and steuartry (except for some shires which were to take turns).
[7] The Representation of the People (Scotland) Act 1868 abolished the rights of the counties of Peebles and Selkirk to return a member, and provided that those counties should jointly return a member, thereby establishing the Peebles and Selkirk constituency.
Mackenzie was appointed a Lord Commissioner of the Treasury, requiring a by-election.