 Marshall was a Liberal Party councillor on Wallasey Borough Council 1963-65 and parliamentary candidate for Louth in 1964 and 1966, but joined the Labour Party in 1967.In a 1971 by-election, Marshall was elected as the Member of Parliament (MP) for Goole.From 1976–9, he served as Parliamentary Private Secretary to the Home Secretary, and as Chair of the Select Committee on Trade and Industry.