Before signing on to write radio shows for Orson Welles' Mercury Theatre, Koch had several plays produced under the auspices of the Federal Theatre Project. His best-known work for Welles was the notorious 1938 adaptation of "The War of the Worlds". With Warner Bros. from 1940, he wrote several outstanding films, notably "The Letter" (1940), "Sergeant York" (1941) and "Letter from an Unknown Woman" (1948), as well as collaborating with the Epstein brothers on the romantic classic "Casablanca" (1942) to which he reputedly brought a political edge by beefing up Rick's political past and romantic penchant for lost causes like fighting on the Loyalists' side in the Spanish Civil War.

Following his blacklisting in 1950, Koch used the pseudonym Peter Howard to write the British-produced "Finger of Guilt" (1956), directed by fellow exile Joseph Losey under the pseudonym Joseph Walton.

Koch retired to Woodstock, NY where he remained actively involved in local theatre, writing and producing plays. He died of pneumonia at age 93 in August 1995. The following December, Koch's Oscar for "Casablanca" was sold at auction by Christie's for $184,000.