An American author and screenwriter, he is most known for such novels as Save the Tiger (1972), The Circle (1982), and Pillars of Fire (1990). He received an Academy Award nomination for his Voyage of the Damned (1977) screenplay.
In his early adulthood, he worked in his family's New York pharmacy. He moved to Hollywood in the late 1950s.
He joined the Coast Guard during World War II and began writing to pass the time.
He was born in Brooklyn to Barnard and Rachel Shagan. He married Elizabeth Ricker in November of 1956.
Jack Lemmon starred in Shagan's 1974 film Save the Tiger.