On coming of age, Macdonald was returned to Parliament for Tain Burghs on the interest of his uncle the 2nd Marquess of Stafford (the marchioness being Countess of Sutherland). He was Member for Tain for only one session, transferring at the general election of 1806 to Newcastle-under-Lyme, on the same interest. Like his patron, he adhered to the Grenville ministry, voting for their repeal of Pitt’s Additional Force Act, 30 Apr. 1806, and being listed a staunch supporter of the abolition of the slave trade. He voted for Brand’s motion against their successors, 9 Apr. 1807. On 15 Apr. he spoke to the same effect on Lyttelton’s motion. His patron had doubts about his interest at Newcastle-under-Lyme, but Macdonald was permitted to stand a contest, in which he narrowly succeeded.
On 22 Jan. 1808 Macdonald dissented from the address at the report stage, with particular reference to the Copenhagen expedition and the omission of peace negotiations. He voted in this sense in February; as well as against the present application of the droits of Admiralty (11 Feb. 1808 and subsequently). He opposed the orders in council, 3 Mar. 1808, the mutiny bill, 14 Mar., was teller against the pensions of the Scotch barons of Exchequer, 4 May, and voted for Catholic relief at every opportunity. On 18 Jan. 1809 he was one of the Whigs who met to endorse George Ponsonby’s leadership in the House. He opposed the convention of Cintra, 21 Feb. 1809, and was in two minorities against the Duke of York on 17 Mar. He supported both motions critical of ministerial corruption, 25 Apr., 11 May 1809, as well as Ward’s motion critical of the Dutch commissioners, 1 May. He was listed ‘thick and thin’ in opposition in 1810 and so behaved on the Scheldt question and on those of (Sir) Francis Burdett and John Gale Jones. He also supported tithe reform, Romilly’s bill to mitigate penalties for private theft, parliamentary reform and sinecure reform that session. On 31 May 1810 he congratulated Henry Bankes on his campaign against sinecures in the face of government hostility. He rallied to opposition on the Regency questions—not without an encouraging hint from the Prince of Wales. He and James Abercromby were in consultation with their leader, Ponsonby, as to tactics in January 1811.
Macdonald (son of the ld. chief baron) spoke well against sinecure places. Fitzgerald said it came with an ill grace from one who himself was a sinecure placeman, different from McMahon in this, that he had not been so from a schoolboy. Macdonald, in reply, called him the l[or]d of the Treasury, who so modestly attacked him, and said he would give up his place when the goodly example was set him by others. We said across the House, why don’t you set it?
Macdonald, whose place was worth £350 p.a., said no more for the rest of the session, but voted against ministers to the bitter end. On Stuart Wortley’s motion his authority as ‘the great list-maker’ was cited as a forecast of its success. He was one of the young Whigs who were anxious for a junction with Canning at that time.
In 1812 Macdonald, in accordance with a suggestion made by the party managers some three years before, transferred his Membership to Sutherland, on the same interest: it was believed that he would have been beaten at Newcastle-under-Lyme, where his patron’s heir replaced him. He was in the minority on the bank-note bill, 11 Dec. 1812, seconded Bankes’s opposition to (and was teller against) the vice-chancellor bill, 11 Feb., and voted for Burdett’s motion on the Regency, 23 Feb. 1813. He was at this time discontented with Ponsonby’s leadership. He criticized the Duke of Cumberland’s interference in the Weymouth election, 1 Apr., but failed on 7 Apr. to procure a select committee to investigate corruption there by 102 votes to 37. His only other known votes that session were all for Catholic relief, and in the next session only one survives, for Creevey’s motion on East India Company affairs, 17 May 1814. Five days before he had been instructed by his patron not to attend the opposition motion against the blockade of Norway. As this was the second such request in a short time, he sought an explanation and learnt that his patron no longer saw eye to eye with the party leadership, at least not on foreign policy.
In Canning’s phrase, ‘hopes were then entertained of Macdonald’s pliability’; as if to encourage them, he joined Grillion’s Club, which was nonpartisan, and went abroad. Ponsonby complained, 4 Jan. 1815, ‘Macdonald who used to make out our lists and attend to our musters in the House will I presume not be in England at our next meeting and we must think of some other person ...’ The sequel was told by Francis Horner to Lord Grey, 27 Oct. 1815:
[Macdonald] did not return from the continent till near the end of last session and had no opportunity of giving any vote but upon the Duke of Cumberland’s question in which he and Lord Gower voted on opposite sides. But he was so little satisfied with the ambiguous manoeuvring of that family with which he is so nearly connected or with the conclusion to which it so manifestly tended that he took upon himself to explain his own opinions and to desire an explanation of theirs. After some evasion, the correspondence has ended in their accepting his resignation, and he is to take the Chiltern Hundreds on the first day of the session ...
Macdonald has the greater merit for acting in this way that he had formed no political connexion but with Lord Stafford by which he was in the smallest degree pledged to particular opinions; and that during the whole of that period which has put men to so strong a test, from the first overthrow of Buonaparte to the last declaration of war, he was abroad.
Macdonald himself, writing to Lord Grenville on 30 Sept., had explained that the renewal of his qualification for the county had precipitated a showdown with the Staffords (in which his father acquiesced); Lady Stafford had already informed him of her husband’s wish to separate from ‘those who had disapproved the results of the battle of Leipzig’. Grenville, in commiseration, commented that this was the first formal intimation he had received of it.
Macdonald’s Whig friends cast about for a new seat for him in December 1815. Tierney, who realized that a purchase was likely to be beyond Macdonald’s means, regretted that he could not get a vacant one at Winchelsea on Lord Darlington’s interest for him, as he was ‘one of the very few who will condescend to make himself useful’.
Macdonald signed the requisition to Tierney to lead the Whig opposition in the Parliament of 1818; Tierney wished him to ‘take a more active part than he has done’. He was ‘exceedingly affected’ by Sir Samuel Romilly’s suicide:
... Romilly was his intimate friend and Macdonald had been the chairman of his election committee in July last—besides this intimacy, there was a coincidence in their ill fortunes—Macdonald had lost his wife at Cowes in the Isle of Wight last year after a lingering illness similar to that which had carried off Lady Romilly at the same place.
On 21 Jan. 1819, according to plan, Macdonald assailed the address in the House, without moving an amendment: ‘a manifesto of the views and intentions of Opposition’, which was ‘uniformly strongly condemned by Tories and applauded by Whigs’.
In the summer of 1819 Macdonald, who had in May been ‘dying for love of Lady Mary Stanley’ for whom Lord Derby thought him too old, so far recovered as to marry Lady Sophia Keppel. His parliamentary patron was abroad and among the Norfolk Whigs he resolved to resist stoutly impending government measures against sedition: the Whigs alone could keep radicalism at bay by ‘a conciliatory system of administration’.