He portrayed Joseph Goebbels, Adolf Hitler's propaganda minister, five times, and also appeared as an SS trooper and a concentration camp officer.

He spent six years in the Max Reinhardt Dramatic School, particularly excelling in Shakespearian roles, and working in revues and musicals in Berlin.

At the age of 23, he appeared in his first film, a silent movie directed by Johannes Brandt called Der Fahnenträger von Sedan.

While Kosleck was acting in The Merchant of Venice on Broadway, Anatole Litvak invited him to Hollywood for a role in a Warner Bros. film.

Kosleck, in a small role playing Nazi Propaganda Minister Joseph Goebbels, revealed a sinister streak of evil that was sought after in wartime movies to come.

He appeared in numerous anti-Nazi films of the early 1940s: Nurse Edith Cavell, Espionage Agent, Underground, Berlin Correspondent, Bomber's Moon, and Chetniks!

Aside from acting, Kosleck was an accomplished painter who supported himself through his work as a portrait artist while waiting for a movie role.

In 1947, Kosleck married, in an unexpected act, the German actress Eleonora von Mendelssohn [de] (her only film is Black Hand, 1950), who committed suicide in 1951.