Gary hails from London, England. His father left the family when Gary was only seven. Gary was raised by his mother and his two sisters. He dropped out of school at the age of sixteen and started working at a sports-equipment store.
After being rejected for admission into Britain's Royal Academy of Dramatic Art, he won a scholarship to the Rose Bruford College, where he received a BA in Drama in 1979. Gary studied with the Greenwich Young People's Theatre and went on to appear in a number of plays throughout the early 80s.
Throughout his career, Gary has become well-known for playing eccentrics and for his mastery of accents. Gary's characters are usually borderline psychotics.
In 1997, he directed, produced, and wrote Nil by Mouth, partially based on his life, which went on to win the Alexander Korda Award for Best British Film, a BAFTA Award and also the British Academy Award for Best Screenplay, the Channel 4 Director's Award, and the Empire Award, and was declared by the British Academy of Film and Television Arts as one of the one hundred best films of all time. Recently Nil By Mouth was listed by Time Out as one of the top two of the top fifty best British films ever.
Gary has been married four times. His first marriage was to Lesley Manville. Together they had one child, Alfie, born in 1988. His second wife was actress Uma Thurman to whom he was married from 1990 till 1992. His third wife was Donya Fiorentino, an American photographer who he first met at an Alcoholics Anonymous meeting in October 1996. They where married from 1997 to 2001. They have 2 children together: Gulliver Flynn Oldman, born on the 20th of August 1997 and Charlie John, born on the 11th February 1999. Gary gained custody of the children when he divorced Donya. His fourth and current wife as of 2008 is Alexandra Edenborough who is an English jazz singer.