The present Duke is the son of Percy Seymour, 18th Duke of Somerset, and Jane née Thomas (died 2005). His paternal grandmother, Edith Mary Parker, was a daughter of William Parker and Lucinda Steeves, daughter of William Steeves , one of the Fathers of Canadian Confederation .
Early life. Cavendish was the son of William Cavendish (1783–1812) and the Honourable Louisa O'Callaghan (d. 1863). His father was the eldest son of Lord George Cavendish (later created, in 1831, The 1st Earl of Burlington, by the second creation), third son of the 4th Duke of Devonshire and Lady Charlotte Boyle, daughter of the 3rd Earl of Burlington and 4th Earl of Cork.