He served with the Queen's Bays in the First World War, attaining the rank of Major and gaining the Croix de Guerre.
Mitchison stood for Parliament without success in King's Norton at the 1931 and 1935 elections.
He was the Labour Member of Parliament for Kettering between 1945 and 1964, beating the young incumbent, John Profumo, at the 1945 election.
However, his association with the Left of the Labour Party proved a barrier to ministerial office.
He was given a life peerage, created Baron Mitchison, of Carradale in the County of Argyllshire on 5 October 1964.