Ponsonby is born on March 29, 1713, the second son of Brabazon Ponsonby, 1st Earl of Bessborough. In 1739, he enters the Irish House of Commons for Newtownards, becoming its speaker in 1756. He also serves as First Commissioner of the Revenue and he becomes a member of the Privy Council of Ireland in 1746. In 1761, he is elected for Kilkenny County and Armagh Borough, and sits for the first. In 1768, he stands also for Gowran and Newtownards, and in 1776 for Carlow Borough, but chooses each time Kilkenny County, which he represents until 1783. Subsequently Ponsonby is again returned for Newtownards and sits for this constituency until his death in 1787.
Belonging to one of the great families which at this time monopolizes the government of Ireland, Ponsonby is one of the principal “undertakers,” men who control the whole of the king’s business in Ireland, and he retains the chief authority until George Townshend, 1st Marquess Townshend, becomes lord-lieutenant in 1767. A struggle for supremacy follows between the Ponsonby faction and the party dependent on Townshend, one result of this being that Ponsonby resigns the speakership in 1771.
In 1743, Ponsonby marries Lady Elizabeth Cavendish, daughter of William Cavendish, 3rd Duke of Devonshire, a connection which is of great importance to the Ponsonbys. His older brother, William Ponsonby, 2nd Earl of Bessborough, had married the Duke’s eldest daughter in 1739. His sons, William Ponsonby, 1st Baron Ponsonby of Imokilly, and George Ponsonby, are also politicians of distinction. His daughter Catherine marries Richard Boyle, 2nd Earl of Shannon, and is mother to Henry Boyle, 3rd Earl of Shannon.
John Ponsonby dies on August 16, 1787.