He was the brother of John Ponsonby, 4th Earl of Bessborough and William Ponsonby, 1st Baron de Mauley, and his sister was the "notorious" Lady Caroline Lamb, who married the Prime Minister Viscount Melbourne. He was educated at Harrow. Early career. Ponsonby joined the army as a cornet in January 1800, serving in the 10th Light Dragoons.
Sarah Ponsonby (1755 – 9 December 1831) was orphaned as a child and lived with relatives in Woodstock, County Kilkenny. A daughter of Chambré Brabazon Ponsonby and Louisa Lyons, she was a second cousin of Frederick Ponsonby, 3rd Earl of Bessborough , and thus a second cousin once removed of his daughter Lady Caroline Lamb .
William Cavendish développa un fort intérêt pour les plantes. En 1838, il fut élu président de la Royal Horticultural Society .La banane Cavendish, soit 97 % de celles qui entrent aujourd'hui dans le commerce international , a été baptisée ainsi en son honneur : dès 1836, il en avait acquis un spécimen, qu'il fit pousser dans la grande serre de Chatsworth .
William Ponsonby †, generaal-majoor, bevelhebber 2e cavaleriebrigade (Union brigade) Henry Paget. Hendrik George de Perponcher Sedlnitsky. Thomas Picton.