Marquis of Salisbury. Part of Box 104, Boswell Collection. Robert Arthur Talbot Gascoyne-Cecil, 3rd Marquess of Salisbury (born 3rd February 1830, died 22nd August 1903) was a leader of the Conservative Party from 1881 to 1902 and was Prime Minister for 13 years, serving in that office on three separate occasions. He was the first Prime Minister to hold office in the 20th century and was the last Prime Minister to hold the office from the House of Lords.