John Jakob Mendelson (6 July 1917 – 20 May 1978) was a British Labour Party politician who was the Member of Parliament for Penistone from 1959 until his death.

John Jakob Mendelson was born on 6 July 1917 in Płock, Poland, to a Jewish family, of which he was the only one who survived the Holocaust.^[1][2][3] He was educated in Berlin, and came to Britain to attend the London School of Economics.^[4] He served in the British Army as a captain during World War II, from 1939 to 1945.^[1][3] In 1949, he became a lecturer in political science at the University of Sheffield,^[3] and was vice-president of Sheffield Trades and Labour Council.

Mendelson was elected MP for Penistone, South Yorkshire at a 1959 by-election, and served on the Public Accounts Committee. He was left-wing and a member of the Tribune Group. However, he clashed with some leftists on certain issues, such as the Soviet Union, which he voiced criticism of.^[3] Conversely, others accused him of being too sympathetic to the Soviet Union.^[5]

Mendelson was instrumental in persuading Harold Wilson to contest the Labour Party leadership in 1963, as a candidate of the left.^[3] He also introduced Tony Benn to the radical history of the Diggers and the Levellers, on which Benn drew from the 1970s onwards.^[6] In the 1970s, he opposed the Wilson government's wage freeze policies.^[7]

On foreign policy, Mendelson joined with Richard Crossman in 1959, in fervently opposing any efforts to give West Germany nuclear weapons.^[3] He was a member of the Labour Friends of Israel.^[1] Mendelson was also a staunch critic of American involvement in the Vietnam War and felt that the Wilson government should have been more vocally opposed to US foreign policy.^[3][5] In 1973, Mendelson became a member of the Consultative Assembly of the Council of Europe.^[1]

Mendelson died from a heart attack in London on 20 May 1978, at the age of 60.^[1][2][5] His successor at the subsequent by-election was Allen McKay.