Monash was born in Harlem, New York, in 1917, and grew up in The Bronx. His mother, Rhoda Melrose, acted in silent films. Monash earned a bachelor's degree in journalism from the University of Wisconsin–Madison and a master's degree in education from Columbia University. An aspiring novelist, he rode the rails across the United States, served in the merchant marine, lived as an expatriate in Paris and studied art.
Monash won early acclaim for his writing for television, including his work on the pioneer anthology series Studio One, Suspense and Playhouse 90. He received an Emmy Award for "The Lonely Wizard," a 1957 episode of Schlitz Playhouse of Stars that starred Rod Steiger. Monash wrote and produced the pilot for the TV series The Untouchables (1959), shown in two parts on Desilu Playhouse and edited as a feature film for distribution in Europe. He also wrote some episodes of the 1958–1959 NBC docudrama about the Cold War, Behind Closed Doors, hosted and starring Bruce Gordon.
His film production credits include Butch Cassidy and the Sundance Kid (1969), Slaughterhouse-Five (1972), The Front Page (1974) and Carrie (1976). Monash produced the feature film The Friends of Eddie Coyle (1973), a dark, critically acclaimed crime drama starring Robert Mitchum, and also adapted the George V. Higgins novel for the screen.
Monash wrote the 1979 CBS-TV adaption of All Quiet on the Western Front, a Hallmark Hall of Fame production that received a Golden Globe Award for Best Motion Picture Made for Television. His screenplay for the HBO film Stalin (1992) was nominated for an Emmy Award; and Monash received the Humanitas Prize for his teleplay for the TNT film George Wallace (1997).
He died in 2003.