Shaffer was born to a Jewish family in Liverpool, the son of Reka (born Fredman) and Jack Shaffer, who was an estate agent. He was the twin brother of writer and dramatist Peter Shaffer. He graduated with a law degree from Trinity College, Cambridge.

Shaffer's most notable work was Sleuth (1970), which he adapted for the film version of the same name, and starred Laurence Olivier and Michael Caine. He received Edgar Awards from the Mystery Writers of America for both versions: for Best Play in 1971, and Best Screenplay in 1973. His other major screenplays include the Hitchcock thriller Frenzy (1972) and the British cult thriller The Wicker Man (1973) with whose director, Robin Hardy, Shaffer had previously set up a television production company in the United States, Hardy, Shaffer & Associates. A major theme in Shaffer's work is the playing of psychological games between people. His two best known works - Sleuth and The Wicker Man - both revolve around this idea.

Novels

• How Doth the Little Crocodile? (1952) - co-written with Peter Shaffer, published under the pseudonym "Peter Anthony"
• Withered Murder (1955) - co-written with Peter Shaffer, published under the pseudonym "Peter Anthony"
• Absolution (1979) - based on Shaffer's screenplay for the 1978 film
• The Wicker Man (1979) - co-written with Robin Hardy, based on Shaffer's screenplay

Plays

• The Savage Parade (1963) - Later revised as This Savage Parade in 1987
• Sleuth (1970)
• Murderer (1975)
• Whodunnit (1977) - Originally called The Case Of The Oily Levantine
• Widow's Weeds (1986) - Originally called For Years I Couldn't Wear My Black

Memoir

• So What Did You Expect? (2001)

Screenplays

• Mr. Forbush and the Penguins (1971) - a.k.a. Cry of the Penguins
• Frenzy (1972)
• Sleuth (1972)
• The Wicker Man (1973)
• Murder on the Orient Express (1974) - uncredited rewrite
• Death on the Nile (1978)
• Absolution (1978)
• Evil Under the Sun (1982)
• Appointment With Death (1988) - co-screenplay
• Sommersby (1993) - co-story