William Brabazon Ponsonby, 1st Baron Ponsonby (1744-1806), son of John Ponsonby, was a leading Whig politician, being a member of the Irish, and after 1800, of the British parliament.

In 1806 shortly before his death he was created Baron Ponsonby of Imokilly. Three of his sons were men of note. The eldest was John (c. 1770-1855), who succeeded to the barony and was created a viscount in 1839 he was ambassador at Constantinople from 1832 to 1837 anc at Vienna from 1846 to 1850. The second son was Major-General Sir William Ponsonby (1772-1815), who, after serving n the Peninsular War, was killed at the battle of Waterloo whilst leading a brigade of heavy cavalry. Another son was Richard Ponsonby (1772-1853), bishop of Derry. Sir William Ponsonby's posthumous son William (1816-1861) became 3rd Baron Ponsonby on the death of his uncle John, Viscount Ponsonby; he died childless and was succeeded by his cousin William Brabazon Ponsonby (1807-1866), only son of the bishop of Derry, on whose death the barony of Ponsonby became extinct.

Among other members of this family may be mentioned Major-General Sir Frederick Cavendish Ponsonby (1783-1837), son of the 3rd earl of Bessborough, a soldier who distinguished himself at the battles of Talavera, Salamanca and Vittoria, in the Peninsular War, and was wounded at Waterloo; he was governor of Malta from 1826 to 1835. His eldest son, Sir Henry Frederick Ponsonby (1825-1895), a soldier who served in the Crimean army, is best remembered as private secretary to Queen Victoria from 1870 until a few months before his death.