Parsons was first returned to the Irish parliament for his university by the ‘popular party’, but on his father’s death succeeded him as Member for King’s County. He had acquired a reputation, as a protégé of Henry Flood, for independence and forthright speaking, and although by 1795 he was prepared to support the war against France, he deprecated the severity of government in 1798 and was a prominent opponent of the Act of Union. He at once took his seat at Westminster in 1801 and voted with ministers on the address, 2 Feb., but on 6 Mar. sought to obstruct the Irish martial law continuation bill and on 12 and 16 Mar. was its leading Irish critic, disclaiming animosity towards ministers, but explaining that he had opposed it in Dublin and could not see Ireland left no better off than Botany Bay. On 19 Mar. he was in the minority against the Irish master of the rolls bill.
Parsons was regarded by government as a country gentleman whom they could win over, and after supporting the peace preliminaries at the prime minister’s request, he made it clear in December 1801 that he wished for an official situation. The prospect of being Irish chancellor of the exchequer or chief secretary was apparently held out to him and Addington promised to recommend him to the viceroy, but neglected to do so. By September 1802 the viceroy concluded that Parsons, who might be a ‘useful friend’ and ‘a troublesome enemy’, would be well satisfied with a seat at the Irish treasury board, though he also pressed for advancement for his brothers in the church and the law, and for a promotion in the peerage for his uncle, Viscount Oxmantown, to whose title he was by remainder to succeed. In November the chief secretary wrote with tongue in cheek of Parsons as one of the Irish Members he had ‘purposely kept back’ from Westminster, ‘because I did not think it safe that they should be left wandering about London streets at such a moment without anyone to take care of them’.
Parsons contrived to ‘come forward very handsomely’ in support of Pitt’s succeeding administration, defending the additional force bill, 8 June 1804. He was one of the few Irish Members disposed to criticize the immediate abolition of the slave trade, advocating gradual abolition, 7 June 1804, for the sake of the mercantile interest, and calling for the evidence to be printed. On 18 June Lord Hawkesbury informed the viceroy that Parsons was to be given a seat at the Irish treasury board, and added: ‘He would undertake to manage the Irish Members who come over to attend Parliament’.
He retained his office under the Grenville ministry which also promoted his uncle to the earldom of Rosse. (Under repeated pressure from Parsons, Oxmantown had agreed to this step by 7 Dec. 1805). This goodwill was perhaps prompted by Lady Downshire’s anxiety to obtain Parsons’s support for a friend of the Prince of Wales at the next election for Longford. But in May 1806 Parsons persisted in remaining in Ireland on account of his wife’s illness when his attendance was required, and was, quite wrongly, suspected of bad faith. Lord Grenville commented: ‘it will never do to have our lords of the Treasury holding off from us in difficulties’. Nevertheless the Castle discouraged an opposition to Parsons at the ensuing general election and he succeeded to his uncle’s title, 20 Apr. 1807, without having committed himself to the outgoing ministers and at a time when he was tipped to become Irish chancellor of the exchequer if Foster declined it. In fact, the Portland ministry confirmed him in office and listened sympathetically and with an eye to his three Members in the Commons, to his plea for a representative peerage, which he obtained in 1809, together with a sinecure office. The viceroy described him as ‘a man of character, known to the public, with a large fortune, and great influence in the country. He resides in Ireland, and is much respected.’