The only son of Harold Solomon and Flora Benenson, Peter Benenson was born in London to a Jewish family. His army officer father died when Benenson was aged nine from a long-term injury, and he was tutored privately by W. H. Auden before going to Eton. At the age of sixteen he helped to establish a relief fund with other schoolboys for children orphaned by the Spanish Civil War. He took his mother's maiden name of Benenson as a tribute to his grandfather, the Russian gold tycoon Grigori Benenson, following his grandfather's death.
He enrolled for study at Balliol College, Oxford but World War II interrupted his education. From 1941 to 1945, Benenson worked at Bletchley Park, the British codebreaking centre, in the "Testery", a section tasked with breaking German teleprinter ciphers. It was at this time when he met his first wife, Margaret Anderson. After demobilisation in 1946, Benenson began practising as a barrister before joining the Labour Party and standing unsuccessfully for election. He was one of a group of British lawyers who founded JUSTICE in 1957, the UK-based human rights and law reform organisation. In 1958 he converted to Roman Catholicism. The following year he fell ill and moved to Italy in order to convalesce.
In 1961 Benenson was shocked and angered by a newspaper report of two Portuguese students from Coimbra sentenced to seven years in prison for raising their glasses in a toast to freedom (this occurred during the autocratic regime of António de Oliveira Salazar). He wrote to David Astor, editor of The Observer. On May 28, Benenson's article, entitled "The Forgotten Prisoners," was published. The letter asked readers to write letters showing support for the students. To co-ordinate such letter-writing campaigns, Amnesty International was founded in Luxembourg in July at a meeting of Benenson and six other men. The response was so overwhelming that within a year groups of letter-writers had formed in more than a dozen countries.
Initially appointed general secretary of Amnesty International, Benenson stood down in 1964 owing to ill health. The advisory position of president of the International Executive was then created for him. In 1966, he began to make allegations of improper conduct against other members of the executive. An inquiry was set up which reported at Elsinore in Denmark in 1967. The allegations were rejected and Benenson resigned from organization.
While never again active in the organization, Benenson was later personally reconciled with other executives, including Seán MacBride. He died in 2005 at the John Radcliffe Hospital, Oxford at the age of 83.