Runciman was a member of the House of Lords as a hereditary peer from the time he inherited the viscountcy on 1 September 1989. He spoke 26 times in the chamber until 11 November 1999 when he lost his right to sit there when the bulk of the hereditary peers were removed by the House of Lords Act 1999. Runciman sat for a subsequent by-election to the Lords in 2010 to fill the Crossbench hereditary seat vacancy following the death of the Viscount Colville of Culross; the House seat went to the Earl of Clancarty.