Ponsonby was educated at Trinity College, Cambridge. He represented Cork City between 1764 and 1776 and thereafter Bandonbridge between 1776 and 1783. He was the leader of a powerful family grouping of between ten and fourteen MPs, the second largest in the Irish House of Commons. During the regency crisis of 1788–89, he gave his support to the Prince of Wales in opposition to William Pitt the Younger. As a consequence he was dismissed from the Post Office. Thereafter he permanently aligned himself with Charles James Fox and together with his brother George gathered together the various small groups of Irish whigs into a unified opposition. As with their English counterparts, their ultimate objective was to re-establish the influence of the landowning classes at the expense of the crown. Ponsonby became committed to the cause of Catholic Emancipation, as a means of securing a loyal population at a time of radical agitation and potential foreign invasion. Pitt's coalition with the Portland whigs in July 1794 and Earl FitzWilliam's consequent appointment as Lord Lieutenant of Ireland gave Ponsonby and his allies an opportunity to regain office. He was on the brink of becoming Irish secretary of state and had sat on the Treasury bench. In 1795, however, he appears to have persuaded FitzWilliam to dismiss John Beresford from his post as first commissioner of the revenue on the grounds of alleged corruption, apparently in revenge for earlier political dealings. The subsequent political crisis led in 1795 to FitzWilliam's swift removal from office, Beresford's re-instatement, and to Ponsonby's humiliating return to opposition. Ponsonby was a leading opponent of the union between Ireland and Great Britain. In 1783, he stood for Newtownards and Kilkenny County. He chose the latter constituency and sat for it from 1783 until the Act of Unioncame into force in 1801. He became then part of the Foxite Whig opposition in the Westminster House of Commons, voting against the Addington and Pitt ministries and in favour of the Prince of Wales and Catholic Emancipation. His influence was declining, however, and by 1803 effective leadership of the Irish whigs had passed to his brother George.
Peerage
By the time Fox regained office in 1806 as member of Grenville's Ministry of All the Talents, Ponsonby's health was poor, with the result that his wife urgently pressed his claims for a peerage, arguing that it was merited by his opposition to the Regency Bill and the Union, and by his staunch support for the Foxite whigs at Westminster. As a consequence he was raised swiftly to the peerage of the United Kingdom on 13 March 1806. He was gazetted as 'Baron Ponsonby, of Imokilly in the County of Cork', although other sources generally refer to him as 'Baron Ponsonby of Imokilly'. He died in Seymour Street, London, on 5 November 1806, and was buried in Ireland.
Testimonials
At a personal level Edmund Burke described Ponsonby in a letter to Lord Charlemont as "a manly, decided character, with... a clear and vigorous understanding." He was as interested in sport as he was in politics and was said to have kept 'the best hunting establishment in Ireland' at Bishopscourt, his seat in County Kildare, where it was also reported that he lived 'in the most hospitable and princely style'. In addition, he was easily irritated, especially if his status and pretensions went unacknowledged. Thus, although he took a leading part in creating a whig opposition in Ireland in the 1790s, he overplayed his hand under FitzWilliam, and his effectiveness was thereafter limited.
Family
In 1769 Ponsonby married Louisa Molesworth, 4th daughter of the 3rd Viscount Molesworth, and his second wife, Mary Usher. They had five sons, four of whom were men of note and one daughter.
