Alun Arthur Gwynne Jones was born in Monmouthshire in 1919. He was educated at West Monmouth School and later at University of London School of Slavonic Studies. He was commissioned into the South Wales Borderers in 1940, and remained in the regiment after the war. He was awarded Military Cross in 1957 for his service in Malaysia, and served as an intelligence officer during the Cold War. He rose to the rank of Lieutenant-Colonel, but on being passed over for command of his regiment, retired from the military in 1961 to take up the post of defence correspondent for the Times, and he also consulted for BBC Television on foreign affairs.

In 1964, he entered the House of Lords as a Labour peer, despite never having been a member of the Labour Party, taking the title Baron Chalfont of Llantaram in the County of Monmouthshire, and was appointed Minister of State for Foreign and Commonwealth Affairs. While in the Foreign Office, he worked on nuclear disarmament strategy and on negotiations for Britain's entry to the European Common Market, and visited the Falkland Islands in 1968 in an unsuccessful attempt to persuade the Islanders to accept a closer relationship with Argentina.

Following the defeat of Harold Wilson's government in the 1970 general election, Chalfont concentrated more on journalism, writing for publications including The Guardian and the New Statesman and appearing regualarly on television and radio, as well as writing several books on military history. In 1974, Chalfont resigned the Labour whip, sitting instead as a crossbencher.

Chalfont was married to Mona Mitchell from 1948 until her death in 2008. They had one daughter.