Lord Roper was the Liberal Democrat Chief Whip in the House of Lords until May 2005. From 2008 to 2012 he was Principal Deputy Chairman of Committees of the House of Lords and Chairman of the European Union Committee. He was, from April 1990 until September 1995, the first director of the Institute for Security Studies of Western European Union in Paris.
As John Roper, he was a member of the House of Commons from 1970 to 1983, an Opposition spokesman on defence (1979-81) and Chief Whip of the Social Democratic Party from 1981 to 1983. From 1983 to 1989 he was a senior member of staff of the Royal Institute of International Affairs (Chatham House) and edited its journal "International Affairs".