Benn's mother, Margaret Wedgwood Benn (née Holmes, 1897–1991), was a theologian, feminist and the founder President of the Congregational Federation. She was a member of the League of the Church Militant, which was the predecessor of the Movement for the Ordination of Women; in 1925, she was rebuked by Randall Davidson, the Archbishop of Canterbury, for advocating the ordination of women. His mother's theology had a profound influence on Benn, as she taught him that the stories in the Bible were based around the struggle between the Prophets and the kings and that he ought in his life to support the Prophets over the kings, who had power, as the Prophets taught righteousness.

The third is dedicated to Suffragette Emily Wilding Davison and was placed in the broom cupboard next to the Undercroft Chapel within the Palace of Westminster, where Davison is said to have hidden during the 1911 census in order to establish her address as the House of Commons.

Both of Benn's grandfathers were Liberal Party MPs; his paternal grandfather was John Benn, a successful Politician, MP for Tower Hamlets and later Devonport, who was created a baronet in 1914 (and who founded a publishing company, Benn Brothers), and his maternal grandfather was Daniel Holmes, MP for Glasgow Govan. Benn's contact with leading politicians of the day, dates back to his earliest years. He met Prime Minister Ramsay MacDonald when he was five years old, whom he described as: "A kindly old gentleman [who] leaned over me and offered me a chocolate biscuit. I've looked at Labour Leaders in a funny way ever since." Benn also met former Liberal Prime Minister David Lloyd George when he was twelve, and later recalled that, while still a boy, he once shook hands with Mahatma Gandhi; in 1931, while his father was Secretary of State for India.

Benn was born in London on 3 April 1925. He had two brothers, Michael (1921–1944), who was killed in the Second World War, and David (1928–2017), a specialist in Russia and Eastern Europe. Their father, william Wedgwood Benn, was a Liberal Member of Parliament from 1906 who crossed the floor to the Labour Party in 1928 and was appointed Secretary of State for India by Ramsay MacDonald in 1929, a position he held until the Labour Party's landslide electoral defeat in 1931. william Benn was elevated to the House of Lords with the title of Viscount Stansgate in 1942 – the new wartime coalition government was short of working Labour peers in the upper house. In 1945–46, william Benn was the Secretary of State for Air in the first majority Labour Government.

Benn met Caroline Middleton DeCamp (born 13 October 1926, Cincinnati, Ohio, United States) over tea at Worcester College, Oxford, in 1949 and just nine days after meeting her; he proposed to her on a park bench in the city. Later, he bought the bench from Oxford City Council and installed it in the garden of their home in Holland Park. Tony and Caroline had four children – Stephen, Hilary, Melissa, a feminist Writer, and Joshua – and ten grandchildren. Caroline Benn died of cancer on 22 November 2000, aged 74, after a career as an educationalist.

Benn's father had been created Viscount Stansgate in 1942 when Winston Churchill increased the number of Labour peers to aid political work in the House of Lords; at this time, Benn's elder brother Michael was intending to enter the priesthood and had no objections to inheriting a peerage. However, Michael was later killed in an accident while on active Service in the Second World War, and this left Benn as the heir to the peerage. He made several unsuccessful attempts to renounce the succession.

In July 1943, Benn enlisted in the Royal Air Force as an aircraftman 2nd Class. His father and elder brother Michael (who was later killed in an accident) were already serving in the RAF. He was granted an emergency commission as a pilot officer (on probation) on 10 March 1945. As a pilot officer, Benn served as a pilot in South Africa and Rhodesia. He relinquished his commission with effect from 10 August 1945, three months after the Second World War ended in Europe on 8 May, and just days before the war with Japan ended on 2 September.

After attending Mr Gladstone's day school near Sloane Square, Benn attended Westminster School, and studied at New College, Oxford, where he read Philosophy, Politics and Economics and was elected President of the Oxford Union in 1947. In later life, Benn removed public references to his private education from Who's Who; in 1970 all references to Westminster School were removed; in the 1975 edition his entry stated "Education—still in progress". In the 1976 edition, almost all details were omitted save for his name, jobs as a member of parliament and as a Government Minister, and address; the publishers confirmed that Benn had sent back the draft entry with everything else struck through. In the 1977 edition, Benn's entry disappeared entirely, and when he returned to Who's Who in 1983, he was listed as "Tony Benn" and all references to his education or Service record were removed.

Following the Second World War, Benn worked briefly as a BBC Radio Producer. On 1 November 1950, he was selected to succeed Stafford Cripps as the Labour candidate for Bristol South East, after Cripps stood down because of ill-health. He won the seat in a by-election on 30 November 1950. Anthony Crosland helped him get the seat as he was the MP for nearby South Gloucestershire at the time. Upon taking the oath on 4 December 1950 Benn became "Baby of the House", the youngest MP, for one day, being succeeded by Thomas Teevan, who was two years younger but took his oath a day later. He became the "Baby" again in 1951, when Teevan was not re-elected. In the 1950s, Benn held middle-of-the-road or soft left views, and was not associated with the young left wing group around Aneurin Bevan.

In November 1960, Lord Stansgate died. Benn automatically became a peer, preventing him from sitting in the House of Commons. The Speaker of the Commons, Sir Harry Hylton-Foster, did not allow him to deliver a speech from the bar of the House of Commons in April 1961 when the by-election was being called. Continuing to maintain his right to abandon his peerage, Benn fought to retain his seat in a by-election caused by his succession on 4 May 1961. Although he was disqualified from taking his seat, he was re-elected. An election court found that the voters were fully aware that Benn was disqualified, and declared the seat won by the Conservative runner-up, Malcolm St Clair, who was at the time also the heir presumptive to a peerage.

Benn continued his campaign outside Parliament. Within two years, though, the Conservative Government of the time, which had members in the same or similar situation to Benn's (i.e., who were going to receive title, or who had already applied for writs of summons), changed the law. The Peerage Act 1963, allowing lifetime disclaimer of peerages, became law shortly after 6 pm on 31 July 1963. Benn was the first peer to renounce his title, doing so at 6.22 pm that day. St Clair, fulfilling a promise he had made at the time of his election, then accepted the office of Steward of the Manor of Northstead, disqualifying himself from the House (outright resignation not being possible). Benn returned to the Commons after winning a by-election on 20 August 1963.

In the 1964 Government led by Harold Wilson, Benn was Postmaster General, where he oversaw the opening of the Post Office Tower, then the UK's tallest building, and the creations of the Post Bus Service and Girobank. He proposed issuing stamps without the Sovereign's head, but this met with private opposition from the Queen. Instead, the portrait was reduced to a small profile in silhouette, a format that is still used on commemorative stamps.

Benn also led the government's opposition to the "pirate" radio stations broadcasting from international waters, which he was aware would be an unpopular measure. Some of these stations were causing problems, such as interference to emergency radio used by shipping, although he was not responsible for introducing the Marine Broadcasting Offences Bill when it came before Parliament at the end of July 1966 for its first reading.

Benn supported various LGBT social movements, which were then known as gay liberation; Benn had voted in favour of decriminalisation in 1967. Talking about Section 28 of the 1988 Local Government Act, a piece of anti-gay legislation preventing the "promotion of homosexuality", Benn said:

Earlier in the month, Benn was promoted to Minister of Technology, which included responsibility for the development of Concorde and the formation of International Computers Ltd. (ICL). The period also saw government involvement in industrial rationalisation, and the merger of several car companies to form British Leyland. Following Conservative MP Enoch Powell's 1968 "Rivers of Blood" speech to a Conservative Association meeting, in opposition to Harold Wilson's insistence on not "stirring up the Powell issue", Benn said during the 1970 general election campaign:

By the end of the 1970s, Benn had migrated to the left wing of the Labour Party. He attributed this political shift to his experience as a Cabinet Minister in the 1964–1970 Labour Government. Benn ascribed his move to the left to four lessons:

Benn's philosophy consisted of a form of syndicalism, state planning where necessary to ensure national competitiveness, greater democracy in the structures of the Labour Party and observance of Party Conference decisions. Alongside an alleged twelve Labour MPs, he spent twelve years affiliated with the Institute for Workers' Control, beginning in 1971 when he visited the Upper Clyde Shipyards, arguing in 1975 for the "labour movement to intensify its discussion about industrial democracy".

Labour lost the 1970 election to Edward Heath's Conservatives and upon Heath's application to join the European Economic Community, a surge in left-wing Euroscepticism emerged. Benn "was stridently against membership", and campaigned in favour of a referendum on the UK's membership. The Shadow Cabinet voted to support a referendum on 29 March 1972, and as a result Roy Jenkins resigned as Deputy Leader of the Labour Party.

Harold Wilson resigned as Leader of the Labour Party and Prime Minister in March 1976. Benn later attributed the collapse of the Wilson government to cuts enforced on the UK by global capital, in particular the International Monetary Fund. In the resulting leadership contest Benn came in fourth out of the six cabinet ministers who stood – he withdrew as 11.8% of colleagues voted for him in the first ballot. Benn withdrew from the second ballot and supported Michael Foot; James Callaghan eventually won. Despite not receiving his support in the second and third rounds of the vote, Callaghan kept Benn on as Energy Secretary. In 1976 there was a sterling crisis, and Chancellor of the Exchequer Denis Healey sought a loan from the International Monetary Fund. Underlining a wish to counter international market forces which seemed to penalise a larger welfare state, Benn publicly circulated the divided Cabinet minutes in which a narrow majority of the Labour Cabinet under Ramsay MacDonald supported a cut in unemployment benefits in order to obtain a loan from American Bankers. As he highlighted, these minutes resulted in the 1931 split of the Labour Party in which MacDonald and his allies formed a National Government with Conservatives and Liberals. Callaghan allowed Benn to put forward the Alternative Economic Strategy, which consisted of a self-sufficient economy less dependent on low-rate fresh borrowing, but the AES, which according to opponents would have led to a "siege economy", was rejected by the Cabinet. In response, Benn later recalled that: "I retorted that their policy was a siege economy, only they had the Bankers inside the castle with all our supporters left outside, whereas my policy would have our supporters in the castle with the Bankers outside." Benn blamed the Winter of Discontent on these cuts to socialist policies.

In the Labour Government of 1974 Benn was Secretary of State for Industry and as such increased nationalised industry pay, provided better terms and conditions for workers such as the Health and Safety at Work Act 1974 and was involved in setting up worker cooperatives in firms which were struggling, the best known being at Meriden, outside Coventry, producing Triumph Motorcycles. In 1975 he was appointed Secretary of State for Energy, immediately following his unsuccessful campaign for a "No" vote in the referendum on the UK's continued membership of the European Community (Common Market). Later in his diary (25 October 1977) Benn wrote that he "loathed" the EEC; he claimed it was "bureaucratic and centralised" and "of course it is really dominated by Germany. All the Common Market countries except the UK have been occupied by Germany, and they have this mixed feeling of hatred and subservience towards the Germans".

He was vilified by most of the press while his opponents implied and stated that a Benn-led Labour Government would implement a type of Eastern European socialism, with Edward Heath referring to Benn as "Commissar Benn" and others referring to Benn as a "Bollinger Bolshevik". Despite this, Benn was overwhelmingly popular with Labour Activists in the constituencies: a survey of delegates at the Labour Party Conference in 1978 found that by large margins they supported Benn for the leadership, as well as many Bennite policies.

Benn stood against Denis Healey, the party's incumbent deputy leader, triggering the 1981 Deputy Leadership election, disregarding an appeal from Michael Foot to either stand for the leadership or abstain from inflaming the party's divisions. Benn defended his decision insisting that it was "not about personalities, but about policies". The result was announced on 27 September 1981; Healey retained his position by a margin of barely 1%. The decision of several soft left MPs, including Neil Kinnock, to abstain triggered the split of the Socialist Campaign Group from the left of the Tribune Group. After Argentina invaded the Falkland Islands in April 1982, Benn argued that the dispute should be settled by the United Nations and that the British Government should not send a task force to recapture the islands. The task force was sent, and following the Falklands War, they were back in British control by mid-June. In a debate in the Commons just after the Falklands were recaptured, Benn's demand for "a full analysis of the costs in life, equipment and money in this tragic and unnecessary war" was rejected by Margaret Thatcher, who stated that "he would not enjoy the freedom of speech that he put to such excellent use unless people had been prepared to fight for it".

For the 1983 election Benn's Bristol South East constituency was abolished by boundary changes, and he lost to Michael Cocks in the selection of a candidate to stand in the new winnable seat of Bristol South. Rejecting offers from the new seat of Livingston in Scotland, Benn contested Bristol East, losing to the Conservative's Jonathan Sayeed in June 1983.

Newly elected to a mining seat, Benn was a supporter of the 1984–85 UK miners' strike, which was beginning when he returned to the Commons, and of his long-standing friend, the National Union of Mineworkers leader Arthur Scargill. However, some miners considered Benn's 1977 industry reforms to have caused problems during the strike; firstly, that they led to huge wage differences and distrust between miners of different regions; and secondly that the controversy over balloting miners for these reforms made it unclear as to whether a ballot was needed for a strike or whether it could be deemed as a "regional matter" in the same way that the 1977 reforms had been. Benn also spoke at a Militant tendency rally held in 1984, saying: "The labour movement is not engaged in a personalised battle against individual cabinet ministers, nor do we seek to win public support by arguing that the crisis could be ended by the election of a new and more humane team of ministers who are better qualified to administer capitalism. We are working for a majority labour government, elected on a socialist programme, as decided by conference." This guest appearance was considered one reason why Benn did not become a member of Labour's Shadow Cabinet.

In June 1985, three months after the miners admitted defeat and ended their strike, Benn introduced the Miners' Amnesty (General Pardon) Bill into the Commons, which would have extended an amnesty to all miners imprisoned during the strike. This would have included two men convicted of murder (later reduced to manslaughter) for the killing of David Wilkie, a taxi driver driving a non-striking miner to work in South Wales during the strike.

Two of Benn's children have been active in Labour Party politics. His eldest son Stephen was an elected Member of the Inner London Education Authority from 1986 to 1990. His second son Hilary was a councillor in London, stood for Parliament in 1983 and 1987, and becoming Labour MP for Leeds Central in 1999. He was Secretary of State for International Development from 2003 to 2007, and then Secretary of State for Environment, Food and Rural Affairs until 2010, later serving as Shadow Foreign Secretary (2015–16). This makes him the third generation of his family to have been a member of the Cabinet, a rare distinction for a modern political family in Britain. Benn's granddaughter Emily Benn was the Labour Party's youngest-ever candidate when she failed to win East Worthing and Shoreham in 2010. Benn was a first cousin once removed of the Actress Margaret Rutherford.

Benn stood for election as Party Leader in 1988, against Neil Kinnock, following Labour's third successive defeat in the 1987 general election, losing by a substantial margin, and received only about 11% of the vote. In May 1989 he made an extended appearance on Channel 4's late-night discussion programme After Dark, alongside among others Lord Dacre and Miles Copeland. During the Gulf War, Benn visited Baghdad in order to try and persuade Saddam Hussein to release the hostages who had been captured.

In 1990, Benn was diagnosed with chronic lymphatic leukaemia and given three or four years to live; at this time, he kept the news of his leukaemia from everyone except his immediate family. Benn said: "When you're in parliament, you can't describe your medical condition. People immediately start wondering what your majority is and when there will be a by-election. They're very brutal." This was revealed in 2002 with the release of his 1990–2001 diaries.

In 1991, Benn reiterated his opposition to the European Commission and highlighted an alleged democratic deficit in the institution, saying: "Some people genuinely believe that we shall never get social justice from the British Government, but we shall get it from Jacques Delors. They believe that a good king is better than a bad Parliament. I have never taken that view." This argument has also been used by many on the right-wing Eurosceptic wing of the Conservative Party, such as Daniel Hannan MEP. Jonathan Freedland writes in The Guardian that "For [Tony Benn], even benign rule by a monarch was worthless because the king's whim could change and there'd be nothing you could do about it."

He made public several episodes of audio diaries he made during his time in Parliament and after retirement, entitled The Benn Tapes, broadcast originally on BBC Radio 4. Short series have been played periodically on BBC Radio 4 Extra. A major biography was written by Jad Adams and published by Macmillan in 1992; it was updated to cover the intervening 20 years and reissued by Biteback Publishing in 2011: Tony Benn: A Biography (ISBN 0-333-52558-2). A more recent "semi-authorised" biography with a foreword by Benn was published in 2001: David Powell, Tony Benn: A Political Life, Continuum Books (ISBN 978-0826464156). An autobiography, Dare to be a Daniel: Then and Now, Hutchinson (ISBN 978-0099471530), was published in 2004.

During his final years in Parliament, Benn placed three plaques within the Houses of Parliament. Two are in a room between the Central Lobby and Strangers' Gallery that holds a permanent display about the suffragettes. The first was placed in 1995. The second was placed in 1996 and is dedicated to all who work within the Houses of Parliament.

In 1997, the Labour Party under Tony Blair won the election. Despite later calling Labour under Tony Blair "the idea of a Conservative group who had taken over Labour" and saying "[Blair] set up a new political party, New Labour", Benn's political diaries Free at Last show that Benn was initially somewhat sympathetic to Blair, welcoming a change of government. Benn supported the introduction of the national minimum wage, and welcomed the progress towards peace and security in Northern Ireland (particularly under Mo Mowlam). He was supportive of the extra public money given to public services in the New Labour years but believed it to be under the guise of privatisation. Overall, his concluding judgement on New Labour is highly critical; he describes its evolution as a way of retaining office by abandoning socialism and distancing the party from the trade union movement, adopting a presidentialist style of politics, overriding the concept of the collective ministerial responsibility by reducing the power of the Cabinet, eliminated any effective influence from the annual conference of the Labour Party and "hinged its foreign policy on support for one of the worst Presidents in US history".

Benn strongly objected to the "immoral" bombing of Iraq in December 1998, saying: "Aren't Arabs terrified? Aren't Iraqis terrified? Don't Arab and Iraqi women weep when their children die? Does bombing strengthen their determination? ... Every Member of Parliament tonight who votes for the government motion will be consciously and deliberately accepting the responsibility for the deaths of innocent people if the war begins, as I fear it will."

There are substantial essays on Benn in the Dictionary of Labour Biography by Phillip Whitehead, Greg Rosen (eds), Politicos Publishing, 2001 (ISBN 978-1902301181) and in Labour Forces: From Ernie Bevin to Gordon Brown, Kevin Jefferys (ed.), I.B. Tauris Publishing, 2002 (ISBN 978-1860647437). Michael Moore dedicates his book Mike's Election Guide 2008 (ISBN 978-0141039817) to Benn, with the words: "For Tony Benn, keep teaching us".

He toured with a one-man stage show and appeared a few times each year in a two-man show with folk singer Roy Bailey. In 2003, his show with Bailey was voted 'Best Live Act' at the BBC Radio 2 Folk Awards. In 2002 he opened the "Left Field" stage at the Glastonbury Festival. He continued to speak at each subsequent festival; attending one of his speeches was described as a "Glastonbury rite of passage". In October 2003, he was a guest of British Airways on the last scheduled Concorde FLIGHT from New York to London. In June 2005, he was a panellist on a special edition of BBC One's Question Time edited entirely by a school-age film crew selected by a BBC competition.

In Bristol, where Benn first served as a member of parliament, a number of tributes exist in his honour. A bust of him was unveiled in Bristol's City Hall in 2005. In 2012 Transport House on Victoria Street, headquarters of Unite the Union's regional office, was officially renamed Tony Benn House and opened by Benn himself. As of 2015 he appears, alongside other famous people associated with the city, on the reverse of the Bristol Pound's £B5 banknote.

In a list compiled by the magazine New Statesman in 2006, he was voted twelfth in the list of "Heroes of our Time". In September 2006, Benn joined the "Time to Go" demonstration in Manchester the day before the start of the final Labour Party Conference with Tony Blair as Party Leader, with the aim of persuading the Labour Government to withdraw troops from Iraq, to refrain from attacking Iran and to reject replacing the Trident missile and submarines with a new system. He spoke to the demonstrators in the rally afterwards. In 2007, he appeared in an extended segment in the Michael Moore film Sicko giving comments about democracy, social responsibility and health care, notably, "If we can find the money to kill people, we can find the money to help people."

Benn told the Socialist Review in 2007 that:

Cameron also said in 2008 that, alongside George Orwell's Nineteen Eighty-Four, Benn's Arguments for Democracy was "a very powerful book which makes the important point that we vest power in people who are elected, and that we can get rid of, rather than those we can't".

In 2009 Benn was admitted to hospital and An Evening with Tony Benn, scheduled to take place at London's Cadogan Hall, was cancelled. He performed his show, The Writing on the Wall, with Roy Bailey at St Mary's Church, Ashford, Kent, in September 2011, as part of the arts venue's first Revelation St Mary's Season. In July 2011 Benn was awarded an honorary doctorate from the University of Glamorgan, Wales.

Tony Benn headed the "coalition of resistance", a group with was opposed to the UK austerity programme. In interviews in 2010 with Amy Goodman on Democracy Now! and 2013 with Afshin Rattansi on RT UK, Benn claimed that the actions of New Labour in the leadup to and aftermath of the Iraq War were such that the former Prime Minister Tony Blair should be tried for war crimes. Benn also claimed in 2010 that Blair had lost the "trust of the nation" regarding the war in Iraq.

In 2011 Benn unveiled a plaque in Highbury, North London, to commemorate the Peasants' Revolt of 1381.

In 2013, Tony Benn reiterated his previous opposition to European integration. Speaking to the Oxford Union on the alleged overshadowing of the EU debate by "UKIP and Tory backbenchers", he said:

Benn's funeral took place on 27 March 2014 at St Margaret's Church, Westminster. His body had lain in rest at St Mary Undercroft in the Palace of Westminster the night before the funeral Service. The Service ended with the singing of "The Red Flag". His body was then cremated; the ashes are expected to be buried alongside those of his wife at the family home near Steeple, Essex.