Bethell's parents were william Gladstone Bethell (11 April 1904 – 17 October 1964) and Ann Margaret Frances (née Barlow; died 17 August 1996). His father was a stockbroker who served in the Royal Artillery in the Second World War. His father was the third son of John Bethell, a banker and Liberal Politician, who became 1st Baron Bethell in 1922. His mother was the daughter of Lieutenant Colonel Robert Barlow. His parents divorced in 1946. His mother subsequently remarried three times.

Bethell was educated at Harrow. He trained as a Russian translator during his National Service from 1956 to 1958, and studied Oriental Languages at Pembroke College, Cambridge, specialising in Arabic and Persian. He graduated in 1962, and befriended Polish students in Cambridge. He continued his education at Cambridge as a mature student, and earned his PhD in 1987.

After he graduated, Bethell worked for the Times Literary Supplement from 1962 to 1964, and was a script Editor for the BBC Radio Drama department from 1964 to 1967.

Bethell married twice. He married, firstly, Cecilia Mary Lothian Honeyman on 7 April 1964. She was the daughter of Alexander Honeyman, professor of oriental languages at St Andrews University. They had two sons, James and william, but divorced in 1971; she died in 1977. He remarried in 1992, to Bryony Lea Morgan Griffiths. They had one son, John. They resided in London. He enjoyed playing tennis and poker, and was a member of the Garrick Club and Pratt's.

His father died in 1964, and he inherited the barony on the unexpected early death of his cousin Guy on 2 December 1967. He sat in the House of Lords as a Conservative until the House of Lords Act 1999 removed most hereditary peers from the chamber. He was appointed as a Lord in Waiting (a government whip in the House of Lords) in June 1970, after the 1970 general election.

He suffered from Parkinson's disease in later life, dying at age 69. He was survived by his second wife, and his three sons. He was succeeded in the barony by his eldest son, James Bethell, the 5th Baron Bethell.

Solzhenitsyn later reopened the issue, claiming that he had not authorised a Slovak dissident, Pavel Licko, to give the manuscript to Bethell, and that Licko was a Soviet agent. Bethell rejected those claims, pointing out that Solzhenitsyn had accepted royalties from the publication of the translation over the years. Solzhenitsyn had been completely unknown in the West until Cancer Ward was published, and he won the 1970 Nobel Prize for Literature, primarily for this work, although he was banned by the Soviet government from receiving it until 1974.

Fluent in Russian and Polish, Bethell often translated the works of Russian and Polish Writers into English. After he published a translation of Aleksandr Solzhenitsyn's Cancer Ward, an article by Auberon Waugh in Private Eye in 1971 suggested Bethell had published the work without permission, and had enabled the Soviet authorities to arrest Solzhenitsyn for circulating anti-Soviet propaganda. Bethell won a libel suit, but was forced to resign as a whip in January 1971 to pursue the litigation, and the controversy denied him a place on Edward Heath's list of Conservative candidates to be appointed to the European Parliament. Heath later refused to discuss the matter with him, but government papers released in 2002 under the 30-year rule revealed that Bethell's contacts with people in Communist Russia and Poland were thought to be a security risk.

Bethell's political fortunes changed when Margaret Thatcher became leader of the Conservative Party. He was nominated to become a member of the European Parliament from 1975 to 1979, and sat as an elected MEP for London North West from 1979 to 1994. He set up the "Freedom of the Skies" in 1980, campaigning to force airlines to reduce their prices which he believed were artificially inflated by a cartel. Perceived as too European, he was not re-elected in 1994, but returned to the European Parliament as an MEP for the new regional constituency of London from 1999 to 2003. He was awarded the European People's Party's Schuman Medal in 2003.

Bethell was staunchly anti-communist. In such books as Betrayed, he strongly supported the Anglo-American efforts to overthrow the Communist regimes of Eastern Europe. What Bethell criticised was the execution of such operations, not their goal. He used his European post to campaign for the human rights of dissidents in the Soviet bloc, including Andrei Sakharov and Anatoly Sharansky. He took a leading role in the foundation of the Sakharov Prize, awarded by the European Parliament since 1985. After the fall of Communism, he continued to support critics of the Russian government, such as Vladimir Gusinsky and Alexander Litvinenko. He was also one of the first people to interview Nelson Mandela at Pollsmoor Prison in 1985.

He became a Commander of the Polish Order of Merit in 1991, and received a Russian Presidential Award in 1992. Bethell was the President of the Uxbridge Conservative Association from 1995 to 1999. He was active in the movement to keep Gibraltar British, serving as President of the Friends of Gibraltar's Heritage from 1992 to 2001. For this he received the Freedom of the City of Gibraltar and the Gibraltar Medallion of Honour in 2008. He also opposed the Turkish occupation of northern Cyprus, and was President of the Friends of Cyprus Association from 2001.