In 1968, he became a lecturer at Leicester Polytechnic remaining until 1974, and in 1971 he was elected to Leicester City Council, becoming leader of the council in 1973. He contested the Harborough seat in 1970. In the February general election of 1974 he contested the constituency of Leicester South, and unseated the Conservative MP, Tom Boardman, in the October election that year. He held the seat until his death, with the exception of the period 1983–1987, when he lost the seat to the ConservativeDerek Spencer by seven votes. During his time out of parliament, he worked as a supply teacher and market trader: building up a stronger, more community oriented reputation. Marshall was an assistant whip between 1977 and the end of James Callaghan's government in 1979. In opposition, he was assistant home affairs spokesman from 1982 to 1983, and deputy shadow spokesman on Northern Ireland from 1987 to 1992. In 1992, he voted for Bryan Gould, seen as the leftwing candidate in the leadership election which followed the resignation of Neil Kinnock. The victorious candidate, John Smith, dispensed with Marshall's services, and Marshall's support for Margaret Beckett in the following leadership election did not endear him to Tony Blair. Increasingly out of step with the mainstream of the Labour Party, he concentrated on constituency matters, and rebelled against Blair's government on many occasions, chiefly on matters relating to immigration and education. His constituency work, especially on immigration and benefit problems, won him strong personal support among local voters. Labour party chairman Ian McCartney described him as "a hard-working and dedicated member of parliament who spoke up for his Leicester constituents and did a great deal to help to transform their communities and the opportunities open to them."
Marshall married Shirley Ellis on 9 June 1962 in Sheffield, and they had a son and daughter. They divorced and he married Susan Carter on 15 July 1986, and became step-father to Tracy carter and later had a close relationship with grandson Kasey Carter in Leicester. Marshall died suddenly and unexpectedly of a heart attack, precipitating a by-election dominated by the invasion of Iraq.
