Mary Beard, an only child, was born on 1 January 1955 in Much Wenlock, Shropshire. Her mother, Joyce Emily Beard, was a headmistress and an enthusiastic reader. Her father, Roy Whitbread Beard, worked as an Architect in Shrewsbury. She recalled him as "a raffish public-schoolboy type and a complete wastrel, but very engaging".

Between 1979 and 1983, Beard lectured in Classics at King's College, London; she returned to Cambridge in 1984 as a Fellow of Newnham College and the only female lecturer in the Classics faculty. Rome in the Late Republic, which she co-wrote with Cambridge Historian Michael Crawford, was published the same year.

Beard graduated from Cambridge with a Bachelor of Arts (BA) degree: as per tradition, her BA was later promoted to a Master of Arts (MA Cantab) degree. She remained at Cambridge for her Doctor of Philosophy (PhD) degree: she completed it in 1982 with a doctoral thesis titled The State Religion in the Late Roman Republic: A Study Based on the Works of Cicero.

Beard married Robin Cormack, a classicist and art Historian, in 1985. Their daughter Zoe is a Historian of South Sudan and their son Raphael is a scholar of Egyptian literature.

John Sturrock, Classics Editor of The Times Literary Supplement, approached her for a review and brought her into literary journalism. Beard took over his role in 1992 at the request of Ferdinand Mount. In 1994 she made an early television appearance on an Open Media discussion for the BBC, Weird Thoughts, alongside Jenny Randles and James Randi among others.

In 2000, Beard revealed in an essay reviewing a book on rape that she too had been raped, in 1978. Her blog, A Don's Life, gets about 40,000 hits a day, according to The Independent (2013).

Shortly after the 11 September 2001 attacks on the World Trade Center, Beard was one of several authors invited to contribute articles on the topic to the London Review of Books. She opined that many people, once "the shock had faded", thought "the United States had it coming", and that "[w]orld bullies, even if their heart is in the right place, will in the end pay the price". In a November 2007 interview, she stated that the hostility these comments provoked had still not subsided, although she believed it had become a standard viewpoint that terrorism was associated with American foreign policy. By this point she was described by Paul Laity of The Guardian as "Britain's best-known classicist".

In 2004, Beard became Professor of Classics at Cambridge. She was elected Visiting Sather Professor of Classical Literature for 2008–2009 at the University of California, Berkeley, where she delivered a series of lectures on "Roman Laughter". In 2007–2008 Beard gave the Sigmund H. Danziger Jr. Memorial Lecture in the Humanities at the University of Chicago.

In December 2010, on BBC Two, Beard presented Pompeii: Life and Death in a Roman Town, submitting remains from the town to forensic tests, aiming to show a snapshot of the lives of the residents prior to the eruption of Mount Vesuvius in 79 CE. In 2011 she took part in a television series, Jamie's Dream School on Channel 4, in which she taught classics to teenagers with no experience of academic success. Beard is a regular contributor to the BBC Radio 4 series, A Point of View, delivering essays on a broad range of topics including Miss World and the Oxbridge interview.

For BBC Two in 2012 she wrote and presented the three part television series, Meet the Romans with Mary Beard, which concerns how ordinary people lived in Rome, "the world's first global metropolis". The critic A. A. Gill reviewed the programme, writing mainly about her appearance (teeth, hair, and clothes), judging her "too ugly for television". Beard admitted that his attack felt like a punch, but swiftly responded with a counter-attack on his intellectual abilities, accusing him of being part of " the blokeish culture that loves to decry clever women". This exchange became the focus of a debate about older women on the public stage, with Beard saying she looked an ordinary woman of her age and "there are kids who turn on these programmes and see there’s another way of being a woman", without Botox and hair dye. Charlotte Higgins assessed Beard as one of the rare academics who is both well respected by her peers and has a high profile in the media.

In addition, in April 2013, she was named as Royal Academy of Arts professor of ancient literature, a largely honorary post.

In August 2014, Beard was one of 200 public figures who were signatories to a letter to The Guardian expressing their hope that Scotland would vote to remain part of the United Kingdom in September's referendum on that issue. In July 2015, Beard endorsed Jeremy Corbyn's campaign in the Labour Party leadership election. She said: "If I were a member of the Labour Party, I would vote for Corbyn. He actually seems to have some ideological commitment, which could get the Labour Party to think about what it actually stands for."

In December 2015 she was again a panelist on BBC's Question Time from Bath.

2016 saw Beard present Pompeii: New Secrets Revealed with Mary Beard on BBC One in March. While May 2016, brought about a four-part series shown on BBC Two, titled Mary Beard's Ultimate Rome: Empire Without Limit.

Beard's standalone documentary Julius Caesar Revealed was shown on BBC One in February 2018. In March, she wrote and presented "How Do We Look?" and "The Eye of Faith", two of the nine episodes in Civilisations, a reboot of the 1969 series by Kenneth Clark.

In Beard's first year she found that some men in the university still held very dismissive attitudes regarding the academic potential of women, which only strengthened her determination to succeed. She also developed feminist views that remained "hugely important" in her later life, although she later described "modern orthodox feminism" as partly cant. Beard has since said that "Newnham could do better in making itself a place where critical issues can be generated" and has also described her views on feminism, saying "I actually can't understand what it would be to be a woman without being a feminist." Beard has cited Germaine Greer's The Female Eunuch, Kate Millett's Sexual Politics, and Robert Munsch’s The Paper Bag Princess as influential on the development of her personal feminism.