Horowitz was born in Stanmore, Middlesex, into a wealthy Jewish family, and in his early years lived an upper-middle class lifestyle. As an overweight and unhappy child, Horowitz enjoyed reading books from his father's library. At the age of 8, Horowitz was sent to Orley Farm, a boarding preparatory school in Harrow, Middlesex. There, he entertained his peers by telling them the stories he had read. Horowitz described his time in the school as "a brutal experience", recalling that he was often beaten by the headmaster. At age 13 he went on to Rugby School, a public school in Rugby, Warwickshire, and discovered a love for writing. Horowitz's mother introduced him to Frankenstein and Dracula. She also gave him a human skull for his 13th birthday. Horowitz said in an interview that it reminds him to get to the end of each story since he will soon look like the skull. From the age of 8, Horowitz knew he wanted to be a writer, realizing "the only time when I'm totally happy is when I'm writing". He graduated from the University of York with a lower second class degree in English literature and art history in 1977, where he was in Vanbrugh College.
In at least one interview, Horowitz claims to believe that H. P. Lovecraft based his fictional Necronomicon on a real text, and to have read some of that text.
Horowitz's father was associated with some of the politicians in the "circle" of prime minister Harold Wilson, including Eric Miller. Facing bankruptcy, he moved his assets into Swiss numbered bank accounts. He died from cancer when his son Anthony was 22, and the family was never able to track down the missing money despite years of trying.
Horowitz now lives in Central London with his wife Jill Green, whom he married in Hong Kong on 15 April 1988. Green produced Foyle's War, the series Horowitz wrote for ITV. They have two sons. He credits his family with much of his success in writing, as he says they help him with ideas and research. He is a patron of child protection charity Kidscape.
Anthony Horowitz's first book, The Sinister Secret of Frederick K Bower, was a humorous adventure for children, published in 1979 and later reissued as Enter Frederick K Bower. In 1981 his second novel, Misha, the Magician and the Mysterious Amulet was published and he moved to Paris to write his third book. In 1983 the first of the Pentagram series, The Devil's Door-Bell, was released. This story saw Martin Hopkins battling an ancient evil that threatened the whole world. Only three of four remaining stories in the series were ever written: The Night of the Scorpion (1984), The Silver Citadel (1986) and Day of the Dragon (1986). In 1985, he released Myths and Legends, a collection of retold tales from around the world.
In between writing these novels, Horowitz turned his attention to legendary characters, working with Richard Carpenter on the Robin of Sherwood television series, writing five episodes of the third season. He also novelised three of Carpenter's episodes as a children's book under the title Robin Sherwood: The Hooded Man (1986). In addition, he created Crossbow (1987), a half-hour action adventure series loosely based on William Tell.
In 1988, Groosham Grange was published. This book went on to win the 1989 Lancashire Children's Book of the Year Award. It was partially based on the years Horowitz spent at boarding school. Its central character is a thirteen-year-old "witch", David Eliot, gifted as the seventh son of a seventh son. Like Horowitz's, Eliot's childhood is unhappy. The Groosham Grange books are aimed at a slightly younger audience than Horowitz's previous books.
This era in Horowitz's career also saw Adventurer (1987) and Starting Out (1990) published. However, the most major release of Horowitz's early career was The Falcon's Malteser (1986). This book was the first in the successful Diamond Brothers series, and was filmed for television in 1989 as Just Ask for Diamond, with an all star cast that included Bill Paterson, Jimmy Nail, Roy Kinnear, Susannah York, Michael Robbins and Patricia Hodge, and featured Colin Dale and Dursley McLinden as Nick and Tim Diamond. It was followed in 1987 with Public Enemy Number Two, and by South by South East in 1991 followed by The French Confection, I Know What You Did Last Wednesday, The Blurred Man and most recently The Greek Who Stole Christmas.
Horowitz wrote many stand-alone novels in the 1990s. 1994's Granny, a comedy thriller about an evil grandmother, was Horowitz's first book in three years, and it was the first of three books for an audience similar to that of Groosham Grange. The second of these was The Switch, a body swap story, first published in 1996. The third was 1997's The Devil and His Boy, which is set in the Elizabethan era and explores the rumour of Elizabeth I's secret son. In 1999, The Unholy Grail was published as a sequel to Groosham Grange. The Unholy Grail was renamed as Return to Groosham Grange in 2003, possibly to help readers understand the connection between the books. Horowitz Horror (1999) and More Horowitz Horror (2000) saw Horowitz exploring a darker side of his writing. Each book contains several short horror stories. Many of these stories were repackaged in twos or threes as the Pocket Horowitz series.
Horowitz began his most famous and successful series in the new millennium with the Alex Rider novels. These books are about a 14-year-old boy becoming a spy, a member of the British Secret Service branch MI6. There are ten books where Alex Rider is the protagonist, and an eleventh is connected to the Alex Rider series (although not part of it) : Stormbreaker (2000), Point Blanc (2001), Skeleton Key (2002), Eagle Strike (2003), Scorpia (2004) Ark Angel (2005), Snakehead (2007), Crocodile Tears (novel) (2009), Scorpia Rising (2011), and the 'connector, Russian Roulette (2013). Horowitz had stated that Scorpia Rising was to be the last book in the Alex Rider series prior to writing Russian Roulette about the life of Yassen Gregorovich., but he has returned to the series with Never Say Die (2017).
In 2003, Horowitz also wrote three novels featuring the Diamond Brothers: The Blurred Man, The French Confection and I Know What You Did Last Wednesday, which were republished together as Three of Diamonds in 2004. The author information page in early editions of Scorpia and the introduction to Three of Diamonds claimed that Horowitz had travelled to Australia to research a new Diamond Brothers book, entitled Radius of the Lost Shark. However, this book has not been mentioned since, so it is doubtful it is still planned. A new Diamond Brothers "short" book entitled The Greek who Stole Christmas! was later released. It is hinted at the end of The Greek who Stole Christmas that Radius of the Lost Shark may turn out to be the eighth book in the series.
In 2004, Horowitz branched out to an adult audience with The Killing Joke, a comedy about a man who tries to track a joke to its source with disastrous consequences. Horowitz's second adult novel, Magpie Murders, is about "a whodunit writer who is murdered while he's writing his latest whodunit". Having previously spoken about the book in 2005, Horowitz expected to finish it in late 2015, and it was published in October 2016.
In August 2005, Horowitz released a book called Raven's Gate which began another series entitled The Power of Five (The Gatekeepers in the United States). He describes it as "Alex Rider with witches and devils". The second book in the series, Evil Star, was released in April 2006. The third in the series is called Nightrise, and was released on 2 April 2007. The fourth book Necropolis was released in October 2008. The fifth and last book was released in October 2012 and is named Oblivion.
For the series The Power of Five, Pentagram required five books for story development, but Horowitz completed only four: The Devil's Door-bell (Raven's Gate), The Night of the Scorpion (Evil Star), The Silver Citadel (Nightrise) and Day of the Dragon (Necropolis). Horowitz was clearly aiming for the same audience that read the Alex Rider novels with these rewrites, and The Power of Five has gained more public recognition than his earlier works, earning number 1 in the top 10 book chart.
In October 2008, Anthony Horowitz's play Mindgame opened Off Broadway at the Soho Playhouse in New York City. Mindgame starred Keith Carradine, Lee Godart, and Kathleen McNenny. The production was the New York stage directorial debut for Ken Russell. Recently he got into a joke dispute with Darren Shan over the author using a character that had a similar name and a description that fitted his. Although Horowitz considered suing, he decided not to.
In March 2009 he was a guest on Private Passions, the biographical music discussion programme on BBC Radio 3.
On 19 January 2011, the estate of Arthur Conan Doyle announced that Horowitz was to be the writer of a new Sherlock Holmes novel, the first such effort to receive an official endorsement from them and to be entitled The House of Silk. It was both published in November 2011 and broadcast on BBC Radio 4. A follow-up novel, Moriarty, was published in 2014.
In October 2014, the Ian Fleming estate commissioned Horowitz to write a James Bond novel, Trigger Mortis, which was released in 2015.
Horowitz was appointed Officer of the Order of the British Empire (OBE) in the 2014 New Year Honours for services to literature.
Horowitz began writing for television in the 1980s, contributing to the children's anthology series Dramarama, and also writing for the popular fantasy series Robin of Sherwood. His association with murder mysteries began with the adaptation of several Hercule Poirot stories for ITV's popular Agatha Christie's Poirot series during the 1990s.
Often his work has a comic edge, such as with the comic murder anthology Murder Most Horrid (BBC Two, 1991) and the comedy-drama The Last Englishman (1995), starring Jim Broadbent. From 1997, he wrote the majority of the episodes in the early series of Midsomer Murders. In 2001, he created a drama anthology series of his own for the BBC, Murder in Mind, an occasional series which deals with a different set of characters and a different murder every one-hour episode.
He is also less-favourably known for the creation of two short-lived and sometimes derided science-fiction shows, Crime Traveller (1997) for BBC One and The Vanishing Man (pilot 1996, series 1998) for ITV. While Crime Traveller received favourable viewing figures it was not renewed for a second season, which Horowitz accounts to temporary personnel transitioning within the BBC. In 2002, the detective series Foyle's War launched, set during the Second World War.
He devised the 2009 ITV crime drama Collision and co-wrote the screenplay with Michael A. Walker.
Horowitz is the writer of a feature film screenplay, The Gathering, which was released in 2003 and starred Christina Ricci. He wrote the screenplay for Alex Rider's first major motion picture, Stormbreaker.