The eight boroughs were controlled by four patrons. In 1754 these were: the Duke of Beaufort (Swansea and Loughor); Herbert Mackworth of Gnoll (Neath and Aberavon); Thomas Talbot of Laycock (Kenfig); and Lord Windsor (Cardiff, Cowbridge, and Llantrisant). The dominant interest was that of Lord Windsor, which passed in 1766 to Lord Mountstuart, who had married Windsor’s daughter and heir.

In 1739, in the absence of a suitable candidate from the families of the other borough patrons, Herbert Mackworth was returned unopposed. He held his seat without a contest until his death, and was succeeded by his son (afterwards Sir Herbert Mackworth). Mackworth lost control of Aberavon to Thomas Mansell Talbot by 1780; and Lord Mountstuart, when his son came of age in 1789, was no longer prepared to acquiesce in his tenure of the seat. Over 100 new freemen were created in Cardiff, and at the general election of 1790 Mackworth was forced to retire in favour of John Stuart.