Like his father, Mackworth was active in developing the industrial enterprises on the Gnoll estate: his main concern was with the Gnoll Copper Company, with works at Neath,
Mackworth controlled two of the Cardiff boroughs, but it was with the additional support of the Windsor interest that he was returned unopposed in 1766 and at all his subsequent elections. In Parliament he was thoroughly independent, and spoke very frequently on a wide variety of subjects. He appears in two of the three extant division lists as having voted with Opposition on the land tax, 27 Feb. 1767, though Newcastle in his list of 2 Mar. classed him as an Administration supporter. In the ensuing Parliament he voted with Opposition on the expulsion of Wilkes, 3 Feb. 1769, and the Middlesex election, 15 Apr. and 8 May, declaring on 3 Feb.:
On 31 Oct. 1776, emphasising his position as an independent country gentleman, he said:
He did not like to hear gentlemen so ready to find a plea for the Americans on every occasion ... He was ever most clearly against the House attempting to tax America as America was not represented in that House; but he thought it highly necessary to maintain that right; and that it was but reasonable America should contribute something in return for the millions she had cost this country. He spoke highly in favour of some of the gentlemen in Opposition, but applauded the ministry.
He does not appear in any of the minority lists, February 1775-December 1778, but in Robinson’s list on the contractors bill, 12 Feb. 1779, he is listed as ‘contra, present, friend’. He voted with Opposition over Keppel, 3 Mar. 1779, and his only other recorded vote during this Parliament was with Opposition on Dunning’s motion, though Robinson in his electoral survey of July 1780 counted him as an Administration supporter. He voted with Administration on Sir James Lowther’s motion against the war, 12 Dec. 1781, and on 14 Dec. told the House that he ‘had supported his Majesty’s ministers in the American war from the conviction they had the real interest of the empire at heart, and that the principle of the war was just, though the issue had been unfortunate’.
In 1790 Mackworth was forced to retire in favour of John Stuart whose father, Lord Mountstuart, was no longer willing to acquiesce in his tenure of the seat. He died 25 Oct. 1791.
