Born in Bath, Somerset, Widdecombe is the daughter of Rita Noreen (née Plummer; 1911-2007) and Ministry of Defence civil servant James Murray Widdecombe. Widdecombe's maternal grandfather, James Henry Plummer, was born to an Irish Catholic family of English descent in Crosshaven, County Cork in 1874. She attended the Royal Naval School in Singapore, and La Sainte Union Convent School in Bath. She then read Latin at the University of Birmingham and later attended Lady Margaret Hall, Oxford, to read Philosophy, Politics and Economics (PPE). She worked for Unilever (1973–75) and then as an administrator at the University of London (1975–87) before entering Parliament.

Over the years, Widdecombe has expressed her support for a reintroduction of the death penalty, which was abolished in the UK in 1965. She notably spoke of her support for its reintroduction for the worst cases of murder in the aftermath of the murder of two 10-year-old girls from Soham, Cambridgeshire, in August 2002, in the Soham murders. She supported the argument that the death penalty would have deterrent value, as within five years of its abolition the national murder rate had more than doubled.

Although she supported the UK's partial decriminalisation of homosexuality in 1967, Widdecombe has consistently opposed LGBT rights while in Parliament, saying in 1999 that "I do not think that [homosexuality] can be promoted as an equally valid lifestyle to [heterosexual] marriage, but I would say the same about irregular heterosexual arrangements." On the issue of an equal age of consent, she said in 2000: "I do not believe that issues of equality should override the imperatives of protecting the young." In 2003, Widdecombe proposed an amendment opposing repeal of Section 28 of the Local Government Act, which banned the promotion of homosexuality by local governments. Out of the 17 parliamentary votes considered by the Public Whip website to concern equal rights for homosexuals, Widdecombe took the opposing position in 15 cases, not being present at the other two votes.

From 1976 to 1978, Widdecombe was a councillor on Runnymede District Council in Surrey. She contested the seat of Burnley in Lancashire in the 1979 general election and then, against David Owen, the Plymouth Devonport seat in the 1983 general election.

She was first elected to the House of Commons in the 1987 general election as member for the constituency of Maidstone (which became Maidstone and The Weald in 1997).

In 1990, following the assassination of the Conservative Politician Ian Gow by the Provisional Irish Republican Army (IRA), the Eastbourne by-election for his seat in the House of Commons was won by the Liberal Democrat David Bellotti. Upon the announcement, Widdecombe told the voters that the IRA would be "toasting their success".

Widdecombe became an Anglican in her 30s, after a period of being an agnostic following her departure from religious schooling. Widdecombe is now a practising Roman Catholic; she converted in 1993 after leaving the Church of England. Her reasons for leaving the latter were many, as she explained to reporters from the New Statesman:

In 1996, Widdecombe, as prisons minister, defended the Government's policy to shackle pregnant prisoners with handcuffs and chains when in hospital receiving ante-natal care. Widdecombe told the Commons the restrictions were needed to prevent prisoners from escaping. "Some MPs may like to think that a pregnant woman would not or could not escape. Unfortunately this is not true. The fact is that hospitals are not secure places in which to keep prisoners, and since 1990, 20 women have escaped from hospitals". Jack Straw, Labour's Home Affairs spokesman at the time, said it was "degrading and unnecessary" for a woman to be shackled at any stage.

In 1997, during the Conservative leadership election of william Hague, Widdecombe spoke out against Michael Howard, under whom she had served when he was Home Secretary. She remarked in the House of Commons that there is "something of the night" about him. The remark was considered to be antisemitic by some and was damaging to Howard, who came last in the first round, an opinion both former ministers share. Howard still became party leader in 2003, and Widdecombe then stated, "I explained fully what my objections were in 1997 and I do not retract anything I said then. But ... we have to look to the Future and not the past."

As an MP, Widdecombe expressed conservative views, including opposition to abortion; it was understood during her time in frontline politics that she would not become Health Secretary as long as this involved responsibility for abortions. Although a committed Christian, she has characterised the issue as one of life and death on which her view had been the same when she was agnostic. Along with John Gummer MP, she converted from the Church of England to the Catholic Church following the decision of the Church of England on the Ordination of women as Priests. In her speech at the 2000 Conservative conference, she called for a zero tolerance policy of prosecution, with the punishment of £100 fines for users of cannabis. This was well received by rank-and-file Conservative delegates.

In 2001, when Michael Portillo was running for leader of the Conservative Party, Widdecombe described him and his allies as "backbiters" due to his alleged destabilising influence under Hague. She went on to say that, should he be appointed leader, she would never give him her allegiance. This was amidst a homophobic campaign led by socially conservative critics of Portillo, who argued that Portillo was only concerned about "the 3 W's [sic] — weed [referring to his support for cannabis legalisation], women and woofters [a derogatory epithet for gay men]".

In 2002, she took part in the ITV programme Celebrity Fit Club. Also in 2002 she took part in a Louis Theroux television documentary, depicting her life, both in and out of politics. In March 2004 she briefly became The Guardian newspaper's agony aunt, introduced with an Emma Brockes interview. In 2005 BBC Two showed six episodes of The Widdecombe Project, an agony aunt television programme. In 2005, she appeared in a new series of Celebrity Fit Club, but this time as a panel member dispensing wisdom and advice to the celebrities taking part. Also in 2005, she presented the show Ann Widdecombe to the Rescue in which she acted as an agony aunt, dispensing no-nonsense advice to disputing families, couples, and others across the UK. In 2005, she also appeared in a discussion programme on Five to discuss who had been England's greatest monarch since the Norman Conquest; her choice of monarch was Charles II.

In the 2005 leadership election, she initially supported Kenneth Clarke again. Once he was eliminated, she turned support towards Liam Fox. Following Fox's subsequent elimination, she took time to reflect before finally declaring for David Davis. She expressed reservations over the eventual winner David Cameron, feeling that he did not, like the other candidates, have a proven track record, and she was later a leading figure in parliamentary opposition to his A-List policy, which she has said is "an insult to women". At the October 2006 Conservative Conference, she was Chief Dragon in a political version of the television programme Dragons' Den, in which A-list candidates were invited to put forward a policy proposal, which was then torn apart by her team of Rachel Elnaugh, Oliver Letwin and Michael Brown.

She was the guest host of news quiz Have I Got News for You twice, in 2006 and 2007. Following her second appearance, Widdecombe vowed she would never appear on the show again because of comments made by panellist Jimmy Carr. She wrote, "His idea of wit is a barrage of filth and the sort of humour most men grow out of in their teens.... [T]here's no amount of money for which I would go through those two recording hours again. At one stage I nearly walked out." She did, however, stand by her appraisal of regular panellists Ian Hislop and Paul Merton, whom she has called "the fastest wits in showbusiness". Merton later revealed that he thought Widdecombe had been "the worst ever presenter" of the show, particularly on her second appearance where Merton claimed she "thought she was Victoria Wood".

In 2007 she awarded the University Challenge trophy to the winners. In the same year, she was cast as herself in "The Sound of Drums", the 12th episode of the third series of the science-fiction drama Doctor Who supporting Mr Saxon, the alias of the Master. Since 2007 Widdecombe has fronted a television series called Ann Widdecombe Versus, on ITV1, in which she speaks to various people about things related to her as an MP, with an emphasis on confronting those responsible for problems she wished to tackle. On 15 August 2007 she talked about prostitution, the next week about benefits and the week after that about truancy. A fourth episode was screened on 18 September 2008 in which she travelled around London and Birmingham talking to girl gangs.

In 2009, Widdecombe appeared with Archbishop John Onaiyekan in an "Intelligence Squared" debate in which they defended the motion that the Catholic Church was a force for good. Arguing against the motion were Stephen Fry and Christopher Hitchens, who won the debate overall.

In October 2010, she appeared on BBC One's Strictly Come Dancing, partnered by Anton du Beke, winning the support of some viewers despite low marks from the judges. After nine weeks of routines strongly flavoured by comedy the couple had received enough support in the public vote to stay in the contest. Widdecombe was eliminated from the competition on Sunday 5 December after the public vote had been combined with the judges' score; she was with Scott Maslen of EastEnders in the bottom two.

Following her retirement, Widdecombe made her stage debut, on 9 December 2011, at the Orchard Theatre, Dartford in the Christmas pantomime Snow White and the Seven Dwarfs, alongside Strictly Come Dancing judge Craig Revel Horwood. In April 2012, she had a ten-minute non-singing cameo part in Gaetano Donizetti's comic opera La Fille Du Regiment, playing the Duchesse de Crackentorp. Widdecombe reprised her pantomime performance, again with Revel Horwood, at the Swan Theatre, High Wycombe in December 2012.

In 2012, Widdecombe hosted a new quiz show for the Sky Atlantic channel, called Cleverdicks. The show ran for one series with 30 one-hour episodes. It featured four contestants, usually high quality members of the UK national quiz circuit and ended with a money round for the winner of each show. In April 2012 Widdecombe presented an hour-long documentary for BBC Radio 5 Live, Drunk Again: Ann Widdecombe Investigates, looking at how the British attitude to alcohol consumption has changed over the last few years. It was revealed in October 2012, that the year's Children in Need's appeal night would feature a Strictly Come Dancing special with former show favourites Russell Grant and Widdecombe. On 4 November 2012, Widdecombe guest-hosted one episode of BBC's Songs of Praise programme about singleness.

In 2010, Widdecombe turned down an offer to be Britain's next ambassador to the Holy See, being prevented from accepting by suffering a detached retina. She was made a Dame of the Order of St. Gregory the Great by Pope Benedict XVI for services to politics and public life on 31 January 2013.

In October 2014, she appeared in the BBC series Celebrity Antiques Road Trip, partnered with expert Mark Stacey, where the pair beat the rival team of Craig Revel Horwood and Catherine Southon.

Widdecombe took part in a television series 24 Hours in the Past, along with Colin Jackson, Alistair McGowan, Miquita Oliver, Tyger Drew-Honey and Zoe Lucker. The four-part series was aired from 28 April–19 May 2015 on BBC One. She took part in an episode of Tipping Point: Lucky Stars in 2016. In 2017, Widdecombe took part in ITV's Sugar Free Farm.

Widdecombe stepped in at short notice to play the Evil Queen in the pantomime Snow White and the Seven Dwarfs at Bridlington Spa in December 2016. She replaced Lorraine Chase; who had been injured in an accident two weeks before rehearsals were due to commence. This was Widdecombe's first appearance as a pantomime 'baddie'; a role she told the press she had always hoped for.

In December 2017 Widdecombe played the Empress of China in the pantomime Aladdin at the Marina Theatre in Lowestoft. The production was the theatre's most successful pantomime to date.

In January 2018, Widdecombe was the first to enter the Celebrity Big Brother house to participate as a housemate in its twenty-first series. A controversial figure in the house, she was criticised over her comments regarding the Harvey Weinstein controversy as well as socially conservative comments to her fellow housemates, most notably drag queen Shane Jenek (Courtney Act). She finished the competition in second place as runner-up to Jenek, who became popular with viewers for challenging Widdecombe's views.