Campbell, who developed in his sheltered boyhood a lifelong interest in the physical sciences, was unhappy at Oxford and persuaded his father to obtain him a commission in his regiment. He served in Ireland in 1798 and in the attack on Holland the following year, but bad health forced him to leave the army in 1801.
In 1799, he replaced his uncle in the family seat for Argyllshire, which he retained without opposition until his retirement. He evidently gave silent support to Pitt and Addington in turn, his uncle having advised his father on the change of administration to make known to the new regime ‘any point’ he had in mind for his sons, ‘who both for the King’s sake and their own must think the weight of your family, if properly applied, of very material consequence’. The duke’s unremitting vigilance over his doings was well-meant but fussy. In March 1801 he was directed not to neglect his parliamentary duties, to return to Inveraray as soon as possible, to show his face at royal functions, to break into the library at Argyll House to rescue some books from damp and to issue medical bulletins on an army of relatives.
After the election of 1802, when Campbell was also unsuccessfully put up by his father for Dunbartonshire, Charles Innes reckoned him completely independent of Dundas, but the Melvillites classed him among those supporters of Addington who would, at the crunch, support Pitt’s return to the head of affairs. He made a bad marriage, which his father, it was later reported, tried to prevent by offering to buy off the young lady with £10,000. The liaison was short-lived, and Campbell, low in health and spirits, decided early in 1803 to go abroad with a medical friend in attendance. When Argyll enjoined him to return by the summer and not to exceed his allowance, he gave submissive assurances on both counts, but his excursion proved more protracted and expensive than his father liked. In Paris he was presented to Talleyrand and Buonaparte. He formed a close friendship with Madame de Staël, but even the warmth of her affection had difficulty in penetrating his introspective gloom. At Baden in July, on the resumption of hostilities, he was almost taken by the French, but escaped disguised as a woman and eventually returned home in September.
On 7 Nov. 1803 he was requested by Addington to attend for the opening of the new session, but it was not until 6 Apr. 1804 that he took his seat.
Campbell voted consistently with opposition for the rest of this period, but his record of attendance was abysmal and he showed hardly any active interest in economical and none in parliamentary reform. His only recorded votes during the life of the Perceval ministry were on the Scheldt inquiry, 5 and 30 Mar. 1810, the Regency bill, 21 Jan. 1811, and for Catholic relief, 24 Apr. 1812. He paired for the division on the orders in council, 3 Mar. 1812. He voted for Catholic relief, 2 Mar., 13 and 24 May 1813, 21 May 1816 and 9 May 1817, but his name appears in only 11 of the other surviving division lists between 1812 and 1820, including those on the blockade of Norway, 12 May 1814, the Duke of Cumberland’s grant, 29 June and 3 July 1815, the property tax, 18 Mar. 1816, Canning’s embassy to Lisbon, 6 May, the renewed suspension of habeas corpus, 23 June 1817, and Tierney’s censure motion, 18 May 1819. He had signed the requisition calling on Tierney to take the Whig leadership in the Commons the previous year. His only known interventions in debate were brief speeches on behalf of General Clavering, 23 Mar. 1809. It is obvious that he had little interest in politics. On 2 Mar. 1813, after a late sitting on the Catholic question, he told a friend that ‘I have as little pleasure in London as can well be imagined, and shall be most happy to find myself in the mail coach with the horses heads to the North’.