Lord Clark joined the Labour Party in 1959 and was first elected as a Member of Parliament (MP) for Colne Valley which he represented from 1970 to 1974. He then became a Senior Lecturer in Politics at Huddersfield Polytechnic (now the University of Huddersfield) until 1979 when he returned to the House of Commons as an MP for South Shields in Tyne and Wear.

Clark resigned as an MP in 2001, and was given a life peerage as Baron Clark of Windermere, of Windermere in the County of Cumbria and now sits in the House of Lords. He was also appointed the paid post of Chairman of the Forestry Commission. He also sits as Chairman for the Lake District National Park Partnership and the Lake District National Park World Heritage Bid.