He was the second son of William Ponsonby, 1st Baron Ponsonby of Imokilly and Hon. Louisa Molesworth. He married Hon. Georgiana FitzRoy, youngest daughter of Charles FitzRoy, 1st Baron Southampton. Between 1796 and 1798, Ponsonby sat as a Member of Parliament (MP) in the Irish House of Commons and represented Bandonbridge. Subsequently he stood for Fethard (County Tipperary) and held this seat until the Act of Union in 1801. He entered the British House of Commons in 1812, sitting for Londonderry until his death. In 1815, he was appointed a Knight Commander of the Order of the Bath (KCB).