 After unsuccessfully standing at Hertford in 1951, Marsh
was elected as Labour Party Member of Parliament (MP) for Greenwich at the 1959 general election.As a backbencher he submitted a private members bill in 1960 which despite Government opposition became the Offices, Shops and Railway Premises Act, a white-collar equivalent of the Factories Act and the forerunner of the Health and Safety at Work Act.When Labour came to power in 1964 he became a Parliamentary Secretary in the Ministry of Labour and subsequently, in 1965, in the new Ministry of Technology.