Cooper was a pioneer in writing for the broadcast media, becoming prolific in both radio and television drama. His early successes included radio dramatisations of Dickens' "Oliver Twist", William Golding's "Lord of the Flies" and John Wyndham's science fiction novel "Day of the Triffids". Wyndham wrote to Cooper congratulating him after the first broadcast. On television he adapted Simenon's Maigret detective novels from the French, which became the major hit of the day (1960–61) starring Rupert Davies as the pipe-smoking sleuth in over 24 episodes, for which he won the Script Award in 1961 of the Guild of Television Producers, which subsequently became BAFTA. He also adapted four Sherlock Holmes stories, Ernest Hemingway's "For Whom the Bell Tolls" (1965), "Les Misérables", Flaubert's "Madame Bovary" and Evelyn Waugh's trilogy of novels "Sword of Honour" (1967) for "Theatre 625". He was more successful in the theatre with his original works as opposed to adaptations. His first full-length play "Never Get Out" was staged at the Edinburgh Festival in 1950 and transferred to the Gate Theatre in London.