Ponsonby was educated at
Pembroke 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 reinstatement, 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
County Kilkenny. He chose the latter constituency and sat for it from 1783 until the
Act of Union came 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==