William Allen was born on 6th January, 1901. He joined the family publishing empire and eventually became chairman of David Allen and Sons.

A member of the Ulster Unionist Party, Allen was elected to the House of Commons in the 1929 General Election. Allen joined the New Party that had been established by Oswald Mosley in 1931. Early supporters included Robert Forgan, John Strachey, John Becket, Harold Nicholson and A. K. Chesterton, but in the 1931 General Election none of the New Party's candidates were elected.

In January 1932, Mosley met Benito Mussolini in Italy. Mosley was impressed by Mussolini's achievements and when he returned to England he disbanded the New Party and replaced it with the British Union of Fascists (BUF). Allen was a major financial backer of the BUF and provided money for setting up a pro-Nazi radio station in Britain. However, later it was discovered that like William Joyce, Allen was providing evidence on the organization to MI5.

On the outbreak of the Second World War Allen left the British Union of Fascists and served in the British Army (1940-42) and worked as a press attaché in Beirut, Baghdad and Ankara (1943-45).

William Allen died on 18th September, 1973.