He started his political career in the Labour Party, having joined in opposition to the Suez conflict in 1956. He served as Parliamentary Secretary (Junior Minister) in the Ministry of Housing and Local Government (under Richard Crossman, then later Anthony Greenwood) where he worked, among much else, on planning and conservation, and on devising the soon-to-be-set up Department of the Environment (Secretary of State, Tony Crosland) He was responsible for setting up the Royal Commission on Environmental Pollution. According to his 1972 publication Preservation he worked on setting up the "Four Towns Report" and played an important role in establishing the foundations of conservation policy through the Town and Country Planning Act 1968 and his 1970 Kennet Report. After the fall of the [first] Wilson Government in 1970, he was appointed Honorary Fellow of the Royal Institute of British Architects, and became Chairman of the Council for the Protection of Rural England, of the Advisory Committee on Pollution of the Sea (ACOPS), and various other organisations. He served as Opposition Spokesman on Foreign Affairs in the House of Lords from 1971 to 1974. He was also a member of the European Parliament, of the Western European Union, and a NATO Parliamentarian. Kennet joined the SDP, serving as Chief Whip of the party in the House of Lords between 1981 and 1983. While in that position, he introduced a bill for the Prohibition of Female Circumcision; it passed on 16 July 1985. Following the decision by the party's membership to merge with the Liberal Party in 1988, Kennet was one of the minority who instead opted to remain with the 'continuing' SDP faction, led by David Owen. He returned to the Labour Party in 1990, soon after the 'continuing' SDP folded, before leaving in opposition to Tony Blair's foreign policy. Under the terms of the House of Lords Act 1999, he lost his automatic right to a seat; he was unsuccessful in the election by the Labour hereditary peers of two of their number to continue to sit after the coming into force of the Act, finishing last in a field of six candidates. In 2005 he sought to return to the House in the by-election among Liberal Democrat hereditary peers caused by the death of Earl Russell; he was unsuccessful, receiving no votes. Until late in life he remained chairman of the Stonehenge Alliance, and an active member of the