Douglas Adams was born to Janet (Donovan) Adams (now Janet Thrift) and Christopher Douglas Adams. His only full-sister of both parents is Susan Adams, born in March 1955. His parents separated and divorced in 1957, and Douglas, Susan and Janet moved in with Janet's parents, the Donovans, in Brentwood, Essex. Douglas's grandmother kept her house as an official RSPCA refuge for hurt animals, which "exacerbated young Douglas's hayfever and asthma."

Christopher Adams remarried in July 1960, to Mary Judith Stewart (born Judith Robertson). From this marriage, Douglas Adams has a half-sister, Heather. Janet remarried in 1964, to a veterinarian called Ron Thrift, providing two more half-siblings to Douglas: Jane and James Thrift.

The Hitchhiker's Guide to the Galaxy
According to Adams, the idea for The Hitchhiker's Guide to the Galaxy occurred to him while he lay drunk in a field in Innsbruck, Austria (though he joked that the BBC would instead claim it was Spain because "It was easier to spell"), gazing at the stars. He had been wandering the countryside while carrying a book called the Hitch-hiker's Guide to Europe when he ran into a town where, as he humorously describes, everyone was either "deaf" and "dumb" or only spoke languages he couldn't. After wandering around and drinking for a while, he went to sleep in the middle of a field and was inspired by his inability to communicate with the townspeople. He later said that due to his constantly retelling this story of inspiration, he no longer had any memory of the moment of inspiration itself, and only remembered his retellings of that moment. A postscript to M. J. Simpson's biography of Adams, Hitchhiker, provides evidence that the story was in fact a fabrication and that Adams had conceived the idea some time after his trip around Europe.

The Hitchhiker's Guide to the Galaxy was originally a six-part (each part titled a "Fit" after Lewis Carroll's The Hunting of the Snark) radio series broadcast weekly in the UK by BBC Radio 4 in March and April 1978. Following the success of the show, another episode was recorded and broadcast, which was commonly known as the Christmas Episode. This had nothing to do with Christmas except in an early draft (which would have had Marvin the Paranoid Android revealed as the "star" followed by the Three Wise Men); it was called the Christmas Episode because it was first broadcast on December 24, 1978 (Christmas Eve). A second series of five fits was broadcast one per night, during the week of the 21 January 1980. The radio programme served as the basis for the first two novels of what eventually became a "trilogy in five parts". It was also the basis for a six-part BBC television series in 1981.

Adams was never a prolific writer and usually had to be forced by others to do any writing. This included being locked in a hotel suite with his editor for three weeks to ensure that So Long, and Thanks For All the Fish was completed. He was quoted as saying, "I love deadlines. I love the whooshing noise they make as they go by."

The books formed the basis for other adaptations, such as three-part comic book adaptations for each of the first three books, an interactive text-adventure computer game, and a photo illustrated edition, published in 1994. This latter edition featured a 42 puzzle designed by Adams, which was later incorporated into paperback covers of all five "Hitchhiker's" novels.

Plans to make HHGG into a major motion picture were in the works for more than twenty years, and were finally freed from development hell in late September 2003. Although Austin Powers director Jay Roach was at one time signed on to the project, the Hammer & Tongs duo, Garth Jennings and Nick Goldsmith, received the responsibility. Key to the go-ahead was a rewrite of the screenplay by Karey Kirkpatrick, who had earlier worked on Chicken Run. Shooting began in spring 2004, with Robbie Stamp, Douglas' friend and business partner, as an executive producer, and Walt Disney Pictures as distributors. Adams once described the Hollywood process as "trying to grill a steak by having a succession of people come into the room and breathe on it."

The BBC has broadcast an adaptation of the final three books in the Hitchhikers series for radio with the surviving members of the original radio cast. The adaptations were done by Dirk Maggs for Above the Title Productions. Maggs had consulted with Adams in 1993 about a third series, which was only realized after Adams's death. The third series, The Tertiary Phase, was broadcast on BBC Radio 4 in September 2004 and is now available on audio CD. Douglas Adams himself can be heard playing the part of Agrajag. So Long, and Thanks For All the Fish and Mostly Harmless made up the fourth and fifth radio series, respectively (on radio they were titled The Quandary Phase and The Quintessential Phase) and these were broadcast in May and June of 2005, and subsequently released on Audio CD. DVD-Audio versions, in 5.1 surround sound, of radio series three, four and five are due to be released by BBC Audio in 2006.

The filming of the movie finished in August 2004. The film was released on 28 April in the United Kingdom, New Zealand and Australia, and on 29 April 2005 in the USA. It was released in other locations in Europe from May through July 2005. The cast includes Sam Rockwell as Zaphod Beeblebrox, Zooey Deschanel as Trillian, Martin Freeman as Arthur Dent, Mos Def as Ford Prefect, Alan Rickman as the voice of Marvin the Paranoid Android, and Stephen Fry as The Book (i.e. the voice of the Guide).