Arthur Ponsonby, the son of Sir Henry Ponsonby (1825–1895), Private Secretary to Queen Victoria, was born in Windsor Castle on 16th February 1871. After being educated at Eton and Balliol College (1890-92), he went abroad to learn German and French and in 1894 entered the Diplomatic Service and worked in Constantinople and Copenhagen before moving to the Foreign Office in 1900.
Ponsonby married Dorothea Parry, the daughter of the composer Charles Hubert Hastings Parry. They set up home at Shulbrede Priory in Linchmere and Dorothea gave birth to Elizabeth (1900) and Matthew (1904).
A member of the Liberal PartyPonsonby resigned from the Foreign Office in 1902 in order to further a career in politics. He served first in the Liberal Central Association office and then in 1906, after defeat at Taunton in the 1906 General Election, was appointed principal private secretary to the prime minister, Henry Campbell-Bannerman. When Campbell-Bannerman died in 1908, Ponsonby won the resulting by-election for the Stirling Burghs. As his biographer, Raymond A. Jones has pointed out: "He achieved notoriety almost immediately after his election by voting against the king's proposed visit to Russia and in consequence found himself excluded from the guest list of the king's garden party. This storm in a teacup established Ponsonby as a radical, opposed to Liberal Imperialism."
While in the House of Commons Ponsonby moved to the left, this was reflected in his books, The Camel and the Needle's Eye (1910) and The Decline of the Aristocracy (1912). Ponsonby became a pacifist and campaigned against an increase in defence spending.
A strong critic of the foreign policy of Herbert Asquith and Sir Edward Grey, Ponsonby was opposed to Britain's involvement in the First World War. Ponsonby joined with Charles Trevelyan, E.D. Morel, George Cadbury, Ramsay MacDonald, Arthur Ponsonby, Arnold Rowntree to form the Union of Democratic Control (UDC). Over the next couple of years the UDC became the leading anti-war organisation in Britain.
Like other anti-war MPs, Arthur Ponsonby was defeated in the 1918 General Election. Ponsonby joined the Labour Party and in the 1922 General Election became the MP for the Brightside division of Sheffield.
In 1925 Ponsonby published a statement of his beliefs, Now is the Time, and launched a petition committing its signatories to "refuse to support or render war service to any Government which resorts to arms". After two years he was able, in a deputation to Stanley Baldwin in December 1927, to present a petition signed by 128,770 people. The following year he published Falsehood in Wartime (1928), which exposed many atrocity stories of the First World War as propaganda lies. Ponsonby insisted "that all disputes between nations are capable of settlement either by diplomatic negotiations or by some form of international arbitration".
After the 1929 General Election, Ramsay MacDonald appointed Ponsonby as Parliamentary Secretary to the Ministry of Transport. The following year Ponsonby was granted a peerage and became Leader of the House of Lords (1930-1935). During this period Ponsonby became the leader of the peace movement in Britain. Martin Ceadel, the author of Pacifism in Britain 1914-1945 (1980) has argued: "The reason for Ponsonby's sudden emergence as the leading British pacifist of the later twenties and early thirties was his belief that he had discovered a new type of pacifism which was both commonsensical in outlook and irrefutable in inspiration."
Arthur Ponsonby joined Richard Sheppard, a canon of St. Paul's Cathedral, to establish the Peace Pledge Union in July 1935. The organization included other prominent religious, political and literary figures including George Lansbury, Vera Brittain, Siegfried Sassoon, Donald Soper, Aldous Huxley, Laurence Housman and Bertrand Russell.
Ponsonby collaborated with George Lansbury in the War Resisters' International, and supported disarmament in opposition to Labour's official policy of collective security. This policy difference led him in September 1935 to resign the leadership of the Labour Party in the House of Lords.
From 1937 the PPU organized alternative Remembrance Day commemorations, including the wearing of white rather than red poppies on 11th November. In 1938 the Peace Pledge Union campaigned against legislation introduced by Parliament for air raid precautions, and the following year against legislation for military conscription.
On the outbreak of the Second World War Ponsonby virtually withdrew from active politics. He resigned from the Labour Party on 15th May 1940 as he opposed to its decision to join the National Government formed by Winston Churchill. Ponsonby suffered an incapacitating stroke in September 1943 from which he never recovered.