McEwan was born in Aldershot, Hampshire, on 21 June 1948, the son of David McEwan and Rose Lilian Violet (née Moore). His father was a working-class Scotsman who had worked his way up through the army to the rank of major.

He met the English undergraduate Allen in the 1970s, while both were at the University of East Anglia; Allen was divorced and had two daughters. According to a Daily Mail interview with Allen in September 2014, she was married when she first met McEwan, and their relationship began after her divorce. They married in 1982: "My first wife was very New Age. I tried to accommodate it," McEwan said in 2009.

McEwan's first published work was a collection of short stories, First Love, Last Rites (1975), which won the Somerset Maugham Award in 1976. He achieved notoriety in 1979 when the BBC suspended production of his play Solid Geometry because of its supposed obscenity. His second collection of short stories, In Between the Sheets, was published in 1978. The Cement Garden (1978) and The Comfort of Strangers (1981) were his two earliest novels, both of which were adapted into films. The nature of these works caused him to be nicknamed "Ian Macabre". These were followed by McEwan's first book for children, Rose Blanche (1985), and a return to literary fiction for The Child in Time (1987), winner of the 1987 Whitbread Novel Award.

McEwan began his career writing sparse, Gothic short stories. The Cement Garden (1978) and The Comfort of Strangers (1981), his first two novels, earned him the nickname "Ian Macabre". These were followed by three novels of some success in the 1980s and early 1990s. His novel Enduring Love (1997) was adapted into an eponymous film. He won the Man Booker Prize with Amsterdam (1998). His following novel, Atonement (2001), garnered acclaim and was adapted into an Oscar-winning film starring Keira Knightley and James McAvoy. This was followed by Saturday (2005), On Chesil Beach (2007), Solar (2010), Sweet Tooth (2012), The Children Act (2014), and Nutshell (2016). In 2011, he was awarded the Jerusalem Prize.

Following The Child in Time, McEwan began to move away from the darker, more unsettling material of his earlier career towards the style that would see him reach a wider readership and gain significant critical acclaim. This new phase began with the publication of the mid-Cold War set espionage drama The Innocent (1990) and Black Dogs (1992), a quasi-companion piece reflecting upon the aftermath of the Nazi era in Europe and the end of the Cold War. McEwan followed these works with his second book for children, The Daydreamer (1994).

The couple divorced in 1995. Allen ended the marriage because she was frustrated by McEwan's "glitterati" associations, and the dissolution of the marriage was followed by a custody dispute over their two teenage sons. Allen absconded to France in 1999 with them (accompanied by her new partner) after McEwan had gained sole custody, although the older boy soon returned to his father to visit Botswana with him. Following a Brittany court ruling that their other son should be returned to his father, McEwan gained a London High Court injunction against his former wife in September of that year preventing her from speaking about the case.

His 1997 novel, Enduring Love, about the relationship between a science Writer and a stalker, was popular with critics, although it was not shortlisted for the Booker Prize. It was adapted into a film in 2004. In 1998, he won the Man Booker Prize for Amsterdam. His next novel, Atonement (2001), received considerable acclaim; Time magazine named it the best novel of 2002, and it was shortlisted for the Booker Prize. In 2007, the critically acclaimed movie Atonement, directed by Joe Wright and starring Keira Knightley and James McAvoy, was released in cinemas worldwide. His next work, Saturday (2005), follows an especially eventful day in the life of a successful neurosurgeon. Saturday won the James Tait Black Memorial Prize for 2005, and his novel On Chesil Beach (2007) was shortlisted for the 2007 Booker Prize. McEwan has also written a number of produced screenplays, a stage play, children's fiction, an oratorio and a libretto titled For You with music composed by Michael Berkeley.

McEwan has been nominated for the Man Booker prize six times to date, winning the Prize for Amsterdam in 1998. His other nominations were for The Comfort of Strangers (1981, Shortlisted), Black Dogs (1992, Shortlisted), Atonement (2001, Shortlisted), Saturday (2005, Longlisted), and On Chesil Beach (2007, Shortlisted). McEwan also received nominations for the Man Booker International Prize in 2005 and 2007.

During the hearing, the judge, Mr Justice Charles, ordered the ruling of Paul Clark, the judge at the custody hearing at Oxford County Court, to be read out. The judge at the Oxford hearing had referred to Allen's "vitriolic campaign" against her ex-husband and also commented: "When thwarted by him [McEwan] or others she has not hesitated to make trouble - witness her 'press releases' in various articles in the press earlier this year [1999]."

In 2002, McEwan discovered that he had a brother who had been given up for adoption during World War II; the story became public in 2007. The brother, a bricklayer named David Sharp, was born six years earlier than McEwan, when their mother was married to a different man. Sharp has the same parents as McEwan but was born from an affair between them that occurred before their parents' marriage. After her first husband was killed in combat, McEwan's mother married her lover, and Ian was born a few years later. The brothers are in regular contact, and McEwan has written a foreword to Sharp's memoir.

In 2006, the Board of Trustees of the Kenyon Review honored McEwan with the Kenyon Review Award for Literary Achievement, writing that "McEwan’s stories, novels, and plays are notable for their fierce artistic dramas, exploring unanticipated and often brutal collisions between the ordinary and the extraordinary."

In 2007, Christopher Hitchens dedicated his book, God Is Not Great, to McEwan.

In 2008, McEwan was among more than 200,000 signatories of a petition to support Italian Journalist Roberto Saviano who received multiple death threats and was placed in police protection after exposing the Mafia-like crime syndicate, Camorra, in his 2006 book Gomorrah. McEwan said he hoped the petition would help "galvanize" the Italian police into taking seriously the "fundamental matter of civil rights and free speech".

In 2009 McEwan joined the 10:10 project, a movement that supports positive action on climate change by encouraging people to reduce their carbon emissions.

In 2010, McEwan received the Peggy V. Helmerich Distinguished Author Award. The Helmerich Award is presented annually by the Tulsa Library Trust.

On 20 February 2011, he was awarded the Jerusalem Prize for the Freedom of the Individual in Society. He accepted the prize, despite controversy and pressure from groups and individuals opposed to the Israeli government. McEwan responded to his critics, and specifically the group British Writers in Support of Palestine (BWISP), in a letter to The Guardian, stating in part, "There are ways in which art can have a longer reach than politics, and for me the emblem in this respect is Daniel Barenboim's West-Eastern Divan Orchestra – surely a beam of hope in a dark landscape, though denigrated by the Israeli religious right and Hamas. If BWISP is against this particular project, then clearly we have nothing more to say to each other." McEwan's acceptance speech discussed the complaints against him and provided further insight into his reasons for accepting the award. He also said he will donate the amount of the prize, "ten thousand dollars to Combatants for Peace, an organisation that brings together Israeli ex-soldiers and Palestinian ex-fighters."

In 2012, the University of Sussex presented McEwan with its 50th Anniversary Gold Medal in recognition of his contributions to literature.

In 2013 as part of a wide-ranging interview with Channel 4 News, McEwan reflected upon the furore that surrounded his remarks on Islamism in 2008, stating "I remember getting a lot of stick five or six years ago saying something disobliging about jihadists. There were voices, particularly on the left, that thought anyone who criticised Islamism was really criticising Islam and therefore racist. Well, those voices have gone quiet because the local atrocities committed by Islamists whether in Pakistan or Mali is so self-evidently vile." In the same interview, McEwan remarked that he felt that protestors of the 2003 Iraq War were "vindicated" by what happened subsequently; argued that the chief legacy of the Iraq War was that "[...] sometimes there are things we could do [before that war] which we no longer can" in foreign affairs; stated that at one point prior to the 2003 invasion he had hoped to be able to seek an audience with Tony Blair to persuade him not to go ahead with the war; and as someone who voted for the Liberal Democrats in the 2010 UK general election, that the current coalition government of the United Kingdom should end, stating "Let's either have a Tory government or let Ed Miliband try something different," to try and turn around a country of "great inequity". McEwan is traditionally a Labour supporter and said he had his "fingers crossed" that Miliband would become Prime Minister.

In October 2014, when he was giving a talk at the Cheltenham Literature Festival to publicise his new novel, The Children Act, Ms Allen in the question and answer session was reported as asking, "When are you going to lift the injunction you have on me and my partner?" She was escorted out of the event by the stewards.

Following the referendum on the United Kingdom's membership of the European Union resulting in a win for the leave or 'Brexit' campaign in June 2016, McEwan wrote a critical opinion article for The Guardian newspaper titled "Britain is changed utterly. Unless this summer is just a bad dream", published 9 July 2016. In the article, McEwan attempted to make sense of the aftermath and consequences of the 'Brexit' vote, noting: " Everything is changed utterly. Or about to be, as soon as your new leader is chosen. The country you live in, the parliamentary democracy that ruled it, for good or bad, has been trumped by a plebiscite of dubious purpose and unacknowledged status. From our agriculture to our science and our universities, from our law to our international relations to our commerce and trade and politics, and who and what we are in the world – all is up for a curious, unequal renegotiation with our European neighbours." McEwan's piece appeared to conclude with a sense of bewilderment and unease at how events were panning out, anticipating the ascension of Theresa May to the leadership of the Conservative Party and her appointment as Prime Minister, and noting how the previously unthinkable in British politics had actually happened. (McEwan's article was published on 9 July, and May effectively won the Conservative Party leadership contest on 11 July, which precipitated her appointment as Prime Minister two days later). In May 2017, speaking at a London conference on Brexit, apparently referring to what he believed to be the older demographic of leave voters, McEwan stated that 'one and a half million oldsters freshly in their graves' would result in a putative second referendum returning a 'remain' outcome.