Knox’s father brought him into the Irish house of commons in 1790 for his own borough of Dungannon and he was considered as belonging to his kinsman Lord Abercorn’s squad of eight members. He showed ‘very good talents’ in debate and was mentioned in 1792 for the provostship of Trinity College, but thought ‘too young’ by the viceroy, who also disliked ‘an unqualified political appointment’. In 1793 Abercorn made him his parliamentary spokesman and obtained him a place at the revenue board. His brother Thomas’s quarrel with Abercorn made no difference to their relations, but when Abercorn withheld support from government, Knox was allowed to vote with them occasionally, lest his place be jeopardized. As Abercorn’s man, although he was not an alumnus, he was chosen for the university at the election of 1797. He equivocated on the question of the Union, though in the last resort he declared that he preferred it to separation. In 1799 Abercorn decided to test his weight by asking government to except Knox from their threat to dismiss all placemen who did not vote for the Union, but Knox relieved government of embarrassment by voluntary resignation and helped to thwart the coalition of anti-Union interests in parliament. As a reward he was promised, as a Union engagement, that he would be restored to his place.
This was not realized because it was now incompatible with a seat at Westminster, and the viceroy Hardwicke cast about for an equivalent worth £1,000 p.a. which was, as Knox later alleged, not even allowing him to take his choice. He refused the offer of the secretaryship to the treasury, 5 Aug. 1801, because he was sure his re-election would be hotly contested by a ‘turbulent demagogue’ and would embarrass Abercorn, if he applied to him to take his seat for his father’s borough of Dungannon, then vacant. He prided himself on serving the government by this renunciation. Though he did not know it, the viceroy was prepared to recommend him in February 1802 as a replacement for Henry Alexander as chairman of committees.
Knox’s re-election in 1802 was indeed hotly contested and his father returned him for Dungannon, in case he failed. His disappointment grew on his return from a visit to Paris. On 4 Mar. 1803 government found his absence from the division lobby ‘dirty’, and when he was subsequently offered the paymastership of corn bounties, his rejection of which office, ripe for abolition, he treated as a foregone conclusion, he found himself placed after all those whose Union claims were based on their votes for it and, in particular, after (Sir) John Stewart, who was promised the next seat at the Irish treasury. Accordingly, Knox became ‘troublesome’; he wrote to the chief secretary, 2 Jan., and to the viceroy, 26 Jan. 1804, reciting his grievances and asking, as a reward for constant support, for the reversion of postmastership-general, though this was inferior in value to his claims; otherwise he feared that, without a specific promise, they would be forgotten. Hardwicke failed to persuade Knox that government had tried to honour their engagement and he was left with the bitter reflection that ‘public service’ was a mere shibboleth to deny him his deserts: ‘the way to court favour is through a labyrinth of which I have not the clue and ... when I think my object is nearest its attainment I find myself in a path which leads me insensibly away’.
Knox voted against Pitt’s second ministry on the additional force bill, June 1804, but was subsequently won over and rewarded with a seat at the Irish treasury board, waived by Sir John Stewart for a quid pro quo, in January 1805. There was a delay owing to an unexpected difficulty about Knox’s re-election, one of his opponents being no less than Foster, the Irish chancellor of the exchequer’s nephew, who was defeated but with difficulty prevailed upon not to press for Knox’s disqualification ex officio, a claim that government confidently rejected.
Deprived of his place and disappointed in the hope that if Abercorn came to terms with government he would be awarded the government of the Cape, Knox went into opposition to the Grenville ministry, voting against the repeal of the Additional Force Act, a measure which he had originally opposed, 30 Apr. 1806.