Kerr was elected to represent Feltham in the 1966 General Election. In the House of Commons Kerr associated with a group of left-wing members that included Konni Zilliacus, Ian Mikardo, Barbara Castle, Sydney Silverman, Geoffrey Bing and Emrys Hughes.
In 1966 Kerr joined Michael Foot, Konni Zilliacus, William Warbey, Anne Kerr, Stan Newens, Norman Atkinson, John Mendelson and Sydney Silverman in protesting against American intervention in Vietnam. However, Kerr and his friends were unable to persuade the Labour prime minister, Harold Wilson, to condemn US policy on Vietnam.
Kerr was chairman of the Tribune Group (1969-70) and the Select Committee on Nationalized Industries (1974-79).