Christie has been referred to by the Guinness Book of World Records as the best-selling writer of books of all time and the best-selling writer of any kind, along with William Shakespeare. Only the Bible is known to have outsold her collected sales of roughly four billion copies of novels. UNESCO states that she is currently the most translated individual author in the world, with only the collective corporate works of Walt Disney Productions surpassing her. Christie's books have been translated into at least 103 languages.

Her stage play The Mousetrap holds the record for the longest initial run in the world: it opened at the Ambassadors Theatre in London on 25 November 1952 and as of 2010 is still running after more than 23,000 performances. In 1955, Christie was the first recipient of the Mystery Writers of America's highest honour, the Grand Master Award, and in the same year Witness for the Prosecution was given an Edgar Award by the MWA for Best Play. Most of her books and short stories have been filmed, some many times over (Murder on the Orient Express, Death on the Nile and 4.50 From Paddington for instance), and many have been adapted for television, radio, video games and comics.

In 1968, Booker Books, a subsidiary of the agri-industrial conglomerate Booker-McConnell, bought a 51 percent stake in Agatha Christie Limited, the private company that Christie had set up for tax purposes. Booker later increased its stake to 64 percent. In 1998, Booker sold its shares to Chorion, a company whose portfolio also includes the literary estates of Enid Blyton and Dennis Wheatley.

In 2004, a 5,000-word story entitled The Incident of the Dog's Ball was found in the attic of the author's daughter. This story was the original version of the novel Dumb Witness. It was published in Britain in September 2009, alongside another newly discovered Poirot story called The Capture of the Cerberus, in John Curran's Agatha Christie's Secret Notebooks: Fifty Years Of Mysteries. On November 10, 2009, Reuters announced that The Incident of the Dog's Ball will be published by The Strand Magazine.

Agatha Mary Clarissa Miller was born in Torquay, Devon, England. Her mother, Clarissa Margaret Boehmer, was the daughter of a British army captain but had been sent as a child to live with her own mother's sister, who was the second wife of a wealthy American. Eventually Margaret married her stepfather's son from his first marriage, Frederick Alvah Miller, an American stockbroker. Thus, the two women Agatha called "Grannie" were sisters. Despite her father's nationality as a "New Yorker" and her aunt's relation to the Pierpont Morgans, Agatha never claimed United States citizenship or connection.

The Millers had two other children: Margaret Frary Miller (1879–1950), called Madge, who was eleven years Agatha's senior, and Louis Montant Miller (1880–1929), called Monty, ten years older than Agatha. Later, in her autobiography, Agatha would refer to her brother as "an amiable scapegrace of a brother".

During the First World War, she worked at a hospital as a nurse; she liked the profession, calling it "one of the most rewarding professions that anyone can follow". She later worked at a hospital pharmacy, a job that influenced her work, as many of the murders in her books are carried out with poison.

Despite a turbulent courtship, on Christmas Eve 1914 Agatha married Archibald Christie, an aviator in the Royal Flying Corps. The couple had one daughter, Rosalind Hicks. They divorced in 1928, two years after Christie discovered her husband was having an affair. It was during this marriage that she published her first novel in 1920, The Mysterious Affair at Styles. In 1924, she published a collection of mystery and ghost stories entitled The Golden Ball.

In late 1926, Agatha's husband Archie revealed that he was in love with another woman, Nancy Neele, and wanted a divorce. On 8 December 1926, the couple quarrelled, and Archie Christie left their house Styles in Sunningdale, Berkshire, to spend the weekend with his mistress at Godalming, Surrey. That same evening Agatha disappeared from her home, leaving behind a letter for her secretary saying that she was going to Yorkshire. Her disappearance caused an outcry from the public, many of whom were admirers of Agatha Christie's novels. Despite a massive manhunt, there were no results until eleven days later.

Eleven days after her disappearance, Christie was identified as a guest at the Swan Hydropathic Hotel (now the Old Swan Hotel) in Harrogate, Yorkshire, where she was registered as 'Mrs Teresa Neele' from Cape Town. Christie gave no account of her disappearance. Although two doctors had diagnosed her as suffering from amnesia, opinion remains divided as to the reasons for her disappearance. One suggestion is that she had suffered a nervous breakdown brought about by a natural propensity for depression, exacerbated by her mother's death earlier that year and the discovery of her husband's infidelity. Public reaction at the time was largely negative, with many believing it was all just a publicity stunt while others speculated she was trying to make the police think her husband killed her as revenge for his affair.