| Constituency | Dates |
|---|---|
| Argyllshire | 5 Feb. 1742 – 1747 |
| Buteshire | 1747 – 1754 |
| Ayr Burghs | 1754 – 1761 |
| Ross-shire | 1761 – 1780 |
Envoy extraordinary, Turin 1758 – 61; P.C. 4 Sept. 1761; ld. privy seal [S] Apr. 1763 – May 1765, Aug. 1766 – d.
Stuart Mackenzie was brought up under the guardianship of his uncles, the 2nd Duke of Argyll and Lord Ilay, later 3rd Duke of Argyll. He was returned for Argyllshire in February 1742 on the interest of the 2nd Duke, with whose followers in the House of Commons he voted against the Government in the following session. He did not vote in the session of 1743-4, when he was abroad with the Barberina, a celebrated opera dancer, with whom he had fallen in love when she came to London for the season of 1742-3. They planned to marry at Venice, but were foiled by Lord Ilay, now Duke of Argyll, who arranged through his friend, Lord Hyndford, the British ambassador at Berlin, for her to be arrested at Venice and sent under escort to Berlin to fulfil a contract which she had made to dance there during the forthcoming season, and for Mackenzie to be deported from Prussia as soon as he arrived in pursuit of her.1John Drummond to Ld. Morton, 4 Dec. 1742, Morton mss. SRO; Letters and Jnls. of Lady Coke, i. pp. lii-iv. On his return to England he is described as ‘ill with love, spitting blood,’ and very angry with Argyll, who wrote: ‘I have no hope for him, for I am told he continues to talk like a madman’.2Walpole to Hanbury Williams, 17 July 1744; Argyll to Andrew Fletcher, Ld. Milton, 27 Oct. 1744, Milton mss. He was absent from the division on the Hanoverians in April 1746, classed by the Government as ‘doubtful’. Returned for Buteshire on his family’s interest in 1747, he married his first cousin, the daughter of the 2nd Duke of Argyll, in 1749. In the next reign he succeeded the 3rd Duke as the manager of affairs in Scotland.
He died 8 Apr. 1800.