John Jakob Mendelson, the son of Polish parents, was born in 1917. After completing his education at the London University he became a factory worker.

Mendelson served in the British Army during the Second World War. After the war he lectured on political science at Sheffield University (1949-59).

A member of the Labour Party he was elected to represent Penistone in the 1959 General Election. In the House of Commons Mendelson associated with a group of left-wing members that included Konni Zilliacus, Ian Mikardo, Barbara Castle, Sydney Silverman, Geoffrey Bing and Emrys Hughes.

In 1965 Mendelson joined Michael Foot, Konni Zilliacus, William Warbey, Russell Kerr, Anne Kerr, Norman Atkinson, Stan Newens, and Sydney Silverman in protesting against American intervention in Vietnam. However, Mendelson and his friends were unable to persuade the Labour prime minister, Harold Wilson, to condemn US policy on Vietnam.

John Jakob Mendelson died on 20th May 1978.