Guinness Book of World Records claims she is the best-selling writer of books of all time and, with William Shakespeare, the best-selling author of any kind. Only the Bible has sold more than her roughly four billion copies of novels. According to UNESCO, Christie is the most translated individual author, with only the collective corporate works of Walt Disney Productions surpassing her. Christie's books have been translated into at least 103 languages.

Christie's stage play The Mousetrap holds the record for the longest initial run: it opened at the Ambassadors Theatre in London on 25 November 1952 and as of 2010 was 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, UK. 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.

Agatha was the youngest of three. 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.

Her first novel, The Mysterious Affair at Styles, came out in 1920. During this marriage, Agatha published six novels, a collection of short stories, and a number of short stories in magazines.

In late 1926, Agatha's husband, Archie, revealed that he was in love with another woman, Nancy Neele, and wanted a divorce. On December 8, 1926, the couple quarreled, 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, she was not found for eleven days.